Nah...Your not Stuck! ....Yet....
Not to be corny, but the biggest thing to do is have FUN. and you did the right thing by starting here. More knowledge around here than...well...than.
You can spend as much or as little on the bike as you want. Depends on how crazy you want to go, and how much time/money/resources you want to put into it. I know that doesn't help much, but check out Hughs Handbuilt and MikesXS for a good idea of parts and costs. And a little bit of elbow grease will go a long way if you put your own time and energy into it.
Unless you have a thing for basket cases, I would try to find a complete, running bike to start, or at least one with motor that has good compression. IMO...that is the only must. If you are going to hack into the frame, I will ask to stay away from the early classics. I think Hugh said it best in one post: there are plenty of ugly ass Specials and heritage's out there to hack into instead of cutting up a classic. If I misquoted/mis-attributed it, please let me know. Either way, I agree. There is a little purist hanging out in me somewhere.
Otherwise (again, IMO) there isn't a single "must have" other than two good tires, a gas tank, a safe frame and a motor that runs. It's your bike, so there isn't a wrong style. Start by looking at every possible XS you can, think about what you like, what you don't, and make if yours. Again: You can't go wrong. You don't even have to mod the frame: build it as a cafe instead. There are a number of picture threads here, otherwise just start googling XS cafe's, XS bobbers, choppers, etc.
You see the guys out in Sturgis with patches that say "I rode mine". It's not so much the fact that they ended up in Sturgis, it was the ride there that they are celebrating. The build here is the same thing: doesn't matter where you end up, as long as your happy with it, because you BUILT it. Most people will never attempt something in their life like this, so savor it =)
Ok...hop off the soap box. I'll let others chime in about common upgrades and such, but like I said, make if yours and have fun!!!