Later cars are easier to get parts for but if you are in California, a 76 car is subject to emissions testing while 75 and earlier are not subject to emissions testing. If you are in any other state you can buy 76 or earlier and probably wont have emissions testing problems.
Square tail light cars are generally cheaper. Almost everything can easily be tinkered with at home except paint and bodywork. Try to find a car with minimal rust. Cutting out rust once it has taken root is a real pain and expensive....
If you are in a hot state, finding a car with a AC console is a plus (Working or not working, doesn't matter, just find the AC console saves a ton of work later)
Tired engines run just fine with a little love, so no need to build the engine right away.