The best advice I can give you is pick something you will enjoy driving for a long time. If you do the work right, you will never get as much for it when you sell it as you put into it.
Also, get something that has a ready parts supply.
That being said, I still have a '73 Alfa Spyder sitting in pieces in my garage. My sticking point has been floorboards. I bought new ones, but haven't managed to weld them in. Now the garage is so full I can't get to the car.
Anyway, all of the suggestions sound fine. GM parts are plentiful, and the cars are relatively easy to work on. Remember, the more exotic, the more $$$$$$!
-Ralphie
