I skipped through some of the thread before posting so apologies if some if this has been covered...
Does it have to be older? I love the looks and raw simplicity of the old sheetmetal from the 50's to early 70's but they're not as impressive performance wise anymore. Most of the power numbers were grossly inflated until about '72 but by that time all the emissions controls hit and everything really sucked hard. Most newer Japanese econobox sedans perform better than most of the old muscle cars, are orders of magnitude more reliable, and fuel efficient.
If you don't know a lot about cars and just want something to drive and enjoy I would highly recommend looking into something newer with fuel injection. By the 1980's fuel injection and better manufacturing standards made vehicles so much more reliable and the drive ability (brakes, chassis etc) is light years ahead of those old cars.
The '86-87 Buick Grand National is a great choice. I just sold mine in December. Depending on the condition they can range from less then $10k to ~$20k for a really low mileage mint one. If you're in the $40-50k range you could easily buy a '97 Buick GNX (only 547 made). Those have pretty much only appreciated in value or at least not gone down lately.
An '88-93 Fox body 5.0L Mustang is another great choice. The sky's the limit with a car like that for modifications and there's tons of them around.
A GMC Scylone or Typhoon would be another really cool choice and you don't see a lot of them anymore.
C4 corvette might a fun option too.
There's a bunch of really cool import options from the 70's-90's as well but i'm assuming you want more american muscle so i'll leave them out for the time being.
I used to love owning older vehicles and working on them. After about 10 years that got old. I like *driving* cars not constantly working on them to sort out all the little problems with carbs, timing and god knows what else that decided to rust out, stop working or just fall off.
It's a tough balance finding something that not *too* nice because you won't want to drive it much to risk putting miles on it and killing the value or just finding something that's a clapped out piece of junk.