Later 70s and 80s cars and trucks are your best bet. I paid $750 for my 1979 Catalina this year, and the only thing it needed is a full brake job and new tires to drive again. Go for the old land yachts, they're generally cheapest. Most of them get turned into demolition derby cars and end up destroyed. 

I almost bought a 1971 Cadillac Sedan Deville with a 400 V8 at an estate auction. It had been garaged and untouched for over 37 years. That only went for $1850. The only reason I didn't bid it up is I have two other classics I need to work on already. Check estate auctions and ask around if you know people who want to get rid of them. My Catalina was my girlfriend's dad's. He wanted it out from in front of the garage. Haha. 

Well you can get some decent 80s ones now if you price around. Especially the trucks