Ok, first off, horses all have different metabolisms, I'm sure you know a person that could eat an entire cheesecake every day and never gain weight and another that so much as looks at a brownie and gains 5 pounds, horses are like that too. You can't just say that horses are designed to eat grass, they are, but, that pretty paint mare that you are looking at is so far removed from her wild roots that it's not even funny.
When looking at hay, especially round bales, it NEEDS to be kept inside, if you leave a round bale outside, it will get wet, the water will sink into the bale and get to the center, then it will dry on the outside and may stay wet in the middle which will produce mold and even possibly a fire as hay gets hot as it molds. The 2% of body weight is a good rule of thumb, but that's exactly what it is, it depends on the horse, activity level, and quality of hay. If the hay has a lot of weeds in it, the horse will need more due to not eating the weeds (you hope, they can be poisoned by eating the wrong weeds).
I don't know where you are, but where I am in Virginia, the rule of thumb is 2 acres PER HORSE and that is if you are dividing it up and rotating. Also, you need to have a sacrifice area which is a dry lot that won't grow anything (eventually, you don't need to kill the grass) and that DOES NOT count in the 2 acres. The sacrifice area is where you feed hay, have the water tubs, and where you keep them in the winter. Unless you have a HUGE pasture, they will absolutely destroy the grass in the winter and it gets weaker every year this happens until the only thing still growing is weeds.
As for grain, you need to ask detailed questions about what she is already eating, brand, how much (should be measured in weight not scoops), any supplements she is on, if she is on supplements, then you need to ask why she is on each one.
Also, unless you are purchasing her from very far away (different climate) ask if they blanket her in the winter, if they do, find out what size blanket she wears and what weight. Some horses turn into wooly mammoths in winter, some barely fluff up, if it gets cold and she doesn't grow much coat, she will burn a TON of calories trying to stay warm and can drop a few hundred pounds in a week unless you are throwing feed at her. When feeding, you should give a maximum of 5 pounds combined feed per meal, if you need to feed more than that, it should be broken up into multiple meals, a horses stomach isn't very large and it works differently than ours. Our stomach will fill up and hold the food until it is digested which gives us a full feeling, horses stomachs empty when they get full even if the food isn't digested yet, they do not get a full feeling and will not stop eating if there is food in front of them, if the grain gets past the stomach without being digested, then it can cause the horse to founder which is huge vet bills and can cause the horse to become un-rideable (and un-breedable due to the added weight when pregnant) or even cause the horse to die despite vet care.
Something you haven't mentioned is manure management. All of that hay comes back out, you need to do something with it. In a LARGE pasture, you can leave it, but the more manure left in the field the greater the chances of getting worms (and DE is not a horse wormer, it MAY help, but I seriously doubt it) but in a small pasture you will need to clean all of the manure up daily. The easiest thing to do is to form a manure pile, but, they can get large FAST, the manure pile attracts flies, and will possibly pollute ground water if you have it were the water table is close to the surface. Also, the manure pile is yet another thing in your field that you cannot use for pasture so it makes the pasture even smaller.
All... Can help point you in the right direction 
