Well, for $250 you might have a lot of trouble getting everything you need. Kits are typically All of the components of the car minus power plant and sometimes radio, along with all of the hardware you'll need. I'm pretty sure you won't find a hobby grade kit that allows you to go all erector-set on it and make something up, though that'd kick ass.
Nitro engines require tuning just like a performance petrol engine. They must be initially broken in and tuned properly, and have that tune maintained. Depending on the engine and the environment you drive it in it may hold it's tune for weeks at a time, or need a tune almost every run. Nitro is typically cheaper to start because you don't have to buy batteries or a charger, but nitro is something like $30/gallon, so it gets expensive. You will get much more run time in a day with nitro unless you have several battery packs and a fast charger.
All of the scratch-built rc's I've seen are rock crawlers, be it scale or otherwise. Well, not
all some people do make some crazy stuff. If you want to build from scratch, you'd better have a lot of patience, fab skill, and be prepared to do a
lot of research as well as trial and error to find out what will work for your build.
Towerhobbies.com is probably the largest online rc retailer, so they should give you a good idea as to what you'll find in your price range, but if something is only a bit out of budget, look around for better prices as tower is not always the best, eBay is great for this.
My recommendation would be to get something used that fits what you want, find the manual(usually downloadable from the manufacturer) and do a full teardown, cleaning and inspection, and rebuild. This will give you a good understanding of how your car goes together and rc kits in general(even if you buy something that's only available as an RTR) as well as the satisfaction of putting it together yourself.
Check out the
forums on the
Radio Control Car Action website. They have a buy/sell/trade forum, though unless you have a reputable eBay account you may have trouble getting sellers to be confident you aren't a non-paying jerk of some sort since you'll be a new and unknown member. I'm a member there as well, though I don't go over too often.
If you want to see some crazy custom builds, check out
rccrawler.com.
Good luck.