I am no expert, but I have done research on raising goats so hopefully I can answer some questions.
@Everose has NDs, and will likely be able to provide much more information.
1. Is $150 too much for 1 intact buckling?
I would say no, but it all depends on your area and quality of the goat. Some can be very expensive.
2. Do I need 2 goats or can I keep just 1?
Goats are herd animals, so you need at least two.
3 Do they need hay all year, or just in winter?
All year, unless you grow it yourself and they're able to graze on fresh grasses.
5. How much space does 1 goat need?
You can have about six goats per acre, but it depends a lot on the land. I recommend rotational grazing, so the goats get to be moved on to fresh grass regularly. It also helps to get them to eat everything in their area rather than just picking a few things to eat.
6. What kind of structure is suitable?
Usually a simple lean-to is all you need. If you ever plan on breeding, an area with a pen that will stay warm is important. A pen of some sort will be useful though. It really depends on your climate.
Goats are very playful and love to climb on things (including your vehicles!), so playground structures are good to have. There's lots of ideas on Pinterest, where people have used tires, pallets, and all sorts of things to make play structures for them.
7. How much dies castration cost?
It depends on your area. On average, it can cost around $100. It's cheaper to just buy a wether.
8. Is it better to castrate them?
Absolutely. Bucks are not fun to deal with, and no one new to goats should get a buckling. They're smelly, can be hard to handle, and are just not fun to be around for most people.
Just remember that they should be castrated at around 12 weeks.
9. Are ND bucks (or wethers) typically aggresive?
Bucks can be aggressive, and dangerous if they have horns. Of course it depends on the individual, but most are going to be a pain.
10. Should I just wait for a doe?
Depends on what you're wanting them for.
Wethers can make good pets, so long as the owner thoroughly researches beforehand.
I would personally wait to get does though.
Something to be aware of is fencing. Goats are escape artists, and are constantly testing their limits. A tall, stury fence that they can't jump over, crawl under, or squeeze through is very important, and can be costly. But is absolutely necessary to protect them from predators and keeping them from getting into trouble.
There are lots of fencing options, some good, some bad, and plenty that are overated.
I highly recommend getting the book "Storey's guide to raising dairy goats". And definitely look into some youtube channels so you can really see what the life of a goat owner is like.