I agree with HP2, if you don't pay now you'll pay later with a stripped down/rusty project car. You'll get nickle and dimed with all the parts. Best to start with a car that might need some body/mechanical work but mostly all there. Something you can fix up in the driveway/school autoshop if there is one that doesn't need major body work (but you never know what you might be able to do in school. Ever see a movie called Corvette Summer? Fiction at best, but inspirational!)
I always like to check out the local craigslist. Local stuff you can see and touch.
Welcome to the site, BTW. Feel free to ask lots of questions. Lots of friendly knowledgeable people here!
Like HP2 said, talk to some people around town like at the garages, auto parts store and get to know some gear head type people that may know if there is a Challenger laying around. If you see a cool old car, talk to the owner and see if he knows of other cool cars in the area. That's how I get to know what's lurking in the local garages. Plus you might get some deals on parts. I got my Magnum 500s for $100 just by asking a guy with a 1969.5 Super Bee if he had some Magnum 500s laying around. They were off his son's Road Runner, hiding in a shed and he didn't need them.
I didn't know there was a awhite 1970 Challenger like a 5 minute drive from my house, been there for many years, neighbor told me about it. Get to know the owners, politely approach them, talk about their car and express some interest in their car and let them know you might be in the market for one of these cars. Who knows, they might give a local kid a break and sell you their car for a decent price.
1970/1971 cars tend to cost more while 1972-1974 cars tend to be less expensive as many options/engine packages were dropped those years. I got my '72 Rallye 340 basically in the condition in the pics (changed the wheels to Magnum 500s) for $4,000 in driver quality but mostly all original form.