If you're young (sounds like you are) and have limited resources (sounds like you do) and want to get an old car to use as a daily driver, I'd discourage you from any import vehicles. Pick a high-production American vehicle and you'll have a better chance of getting replacement and peformance parts as you start tweaking things, with less effort and expense than an import. Camaros and Mustangs have already been beaten to death, try to stay away from an ultra-high demand vehicle. Your selection of Nova or Challenger sounds pretty logical to me, you have a wide selection of performance parts and still have a chance of finding parts in a junkyard without getting absolutely gouged pricewise. If you're lucky you can find an old car as part of an estate sale, Grandpa's old car that no one in the family wants to mess with. You can get really lucky and find someone else's project that they're tired of messing with, but chances are that they've either found a horrendous problem that forces the sale, or will try to ask far more than it's worth in an attempt to recover their previous expenses.
Keep an eye out in your local paper, if they list vehicles by make you're more likely to find a bargain there (i.e. '71 Nova $800) than under a 'Classics' or 'Antiques' listing. Autotrader & similar papers are pretty helpful, too.