I definitely think you are doing the right thing by researching first. That is always a step in the right direction, no matter what you are thinking about.
Some of the others are right...owning a horse is not easy or cheap, but it can be fun and very rewarding. It's great to see new and educated people getting into the hobby the right way, and I am always glad to help them whether its looking at potential horses, giving them tours or information, suggesting books, or referring farms for lessons or work experience. Unfortunately, horse people are a breed of their own and many of them have very strong opinions. It is important to remain open to suggestions, but don't get discouraged if this is something you would really like to do. There are a thousands different ways to have horses and many of those ways are perfectly fine. If you get into the world of horses, you will find your own ways to do things, and you may find some ways that just don't work as well for you. Horse ownership is a neverending learning process. I've been around horses for 16 years and I still learn, every time I am around horses. Unfortunately, college has put a huge dent in the amount of time I get to spend with my horses, but I will always have a love for them.
In general, horses ARE expensive. Shoeing, vet care, and feed can be pricey depending on where you live. Here, shoeing is about $120 per horse, trimming is between $60 and $80 if you keep your horse barefoot. However, I've found trail horses in my state do much better with at least front shoes. Remember, once you begin shoeing a horse, it is much more difficult to go barefoot again. Also, depending on where you live, horses with shoes can be a pain in the neck during the winter and that is something to keep in mind. They tend to get ice balls in their shoes which need to be chipped out. We usually have to do this twice a day when we feed. Luckily only two of our four horses have shoes, but when we get the third mare on the trails we'll have a third horse to worry about. This can be kind of time consuming in addition to feeding.
Someone mentioned horses need to be fed three times a day, but most of the time, unless under certain conditions, a horse only needs feeding twice a day. It takes me about half an hour per feeding at my house and we don't stall our horses. Feeding usually consists of throwing hay over the fence into piles and a scoop of feed each. Now it takes a little longer because our old gelding has to be separated for food and he takes his good old time eating when he doesn't have to rush around other horses. Feeding round bales can really cut down feeding time and in our case hay costs. Otherwise, generally for four horses we are doing about two square bales a day. In the summer less because they eat grass. We buy a bag of feed about once a week, sometimes more often. We supplement with beet pulp. The cost of feeding horses is probably a few hundred a month. It is MUCH easier caring for horses in the summer months though. The winter can be a royal pain in the butt, not only because feeding and care takes longer, but because its really difficult to ride in the winter if you have an open arena. Lack of riding during winter CAN make for a sour horse come spring, so its important to spend time during the winter, even if it's just ground work in between throwing the saddle on a couple times a week. Our more trained horses do not need riding every day, however we find it is helpful to put a saddle on or do some sort of ground work daily with our greener horses. Our seasoned trail horses get most of their workout on the trail and my dad goes out multiple times a week now. Before he retired, they were worked on weekends on the trail and I would take them out after school about three times a week. I certainly saw a difference when I worked with the horse consistently, but for an older, well trained horse, I don't think daily riding is a MUST. There are lots who will disagree and I will say that finding an older (in it's teens) well trained horse will probably not be an easy task. If it IS an easy task, you probably haven't found the perfect horse for you. Horse shopping is actually not as fun as it sounds!
I do have to say I disagree with the other posters who say you cannot keep a horse on five acres. You can certainly keep a pair of horses on five acres if you plan the fencing right. I would certainly plan on having at least two decent sized pastures to rotate during the spring, summer, and fall. The winter is not so important because you are mostly feeding hay. Do be sure to check out the trees in the pasture before putting a horse out there. Some trees are very, very toxic to horses so you'll want to know whats out there. I would plan to have separate pastures from your emus though. In some cases that can work ok as I've seen it done sucessfully down the street from me, but generally I think it would be much, much easier to just have separate pastures.
As far as keeping a single horse, I would not plan on doing that forever. Short term it is ok. The situation is not ideal, but I would not rush to buy two horses just because you feel you need two. Buy your riding horse first, get him or her situated, then look for a companion. Rushing into two horses right away just does not seem like a smart thing to do in my opinion. Single horses in the long run are generally not very happy horses, but for a short time, in my experience, it does not cause harm for most sane horses. Having a single horse for about half a year gave us time to focus on her before we bought a second one. We did have goats when we had a single horse and she spent time with them, but there is not much that can replace the company of another horse. Granted, your second horse does not have to be full sized and that is useful information to keep in mind. However, you should have NO problem finding a compatable older horse for a companion when you need to.
As far as experience goes, I would start with taking lessons and volunteer some time working around the horses. Go to horse expos, seminars, and shows. Talk to horse owners. Many of them will be more than willing to talk to you and give you information. Making connections with other knowledgeable horse owners is a VERY important thing to do. When you go searching for a horse, you want to make sure you bring someone who knows horses along with you. This can either be a trainer or friend, but I would not go look at a potential horse by yourself. You may want to look into leasing, or leasing to buy. There are some awesome horses out there available for lease and it is a smart option for many people. You might also want to look into leasing a horse at a barn, even though you have your own property.
If you have kids and they are interested in horses, see if they might like to join a 4-H club. It may be a club for kids, but I can gaurantee you will learn A LOT as a parent.
Tack sales and fairs are another great place to go and make friends and get information. People are usually very willing to talk about their horses! Just keep in mind you will come across some nasty people, or people who believe they know it all. Its frustrating, but keep an open mind and if they are too pushy, just move on. When it comes to horses you will hear a lot you don't want to hear and some of it is important information. Just read, read, read. Ask questions, and when you think you've heard all you can hear, ask more questions.
I HAVE seen some horror stories, but I don't like to push people away from getting into horses. Like I said, it is a very rewarding hobby. However, it can turn nasty very quickly and I think that's what some people here are trying to prevent. Most everyone on this thread has had some kind of bad experience with horses. It's not a question of "will something happen" but more of WHEN will it happen. Horses are dangerous creatures, but the more knowledgeable you are, the more prepared you will be to deal with situations. Even experienced people still have problems when it comes to horses. I've seen people who claim to know it all and who have had horses for 25+ years have issues with loading a horse on a trailer. Owning horses is a real exercise in patience and that's probably the best word I can think of to describe my experience with horses. It takes a LOT of patience and it can be really frustrating at times. You just have to ask yourself if after all the frustration, will you still love horses enough to be enthusiastic about keeping them. If the answer to that question is "no" or "maybe", horses may not be the animal for you.
If you have more questions, feel free to PM me. I don't claim to know everything about horses, in fact I know relatively little when it comes to the grand scheme of things. However, I can offer 16 years of experience or suggestions on where to find the answer if I cannot fully answer a question. Sixteen years and seven horses later (seven owned at least), it's been a long, heartbreaking, and frustrating road, but I would not trade it for the world!