If you look for the right auctions you should be able to buy all of them for less than 500. If I was to buy just one I would agree with the vectrex. They can be expensive and hard to find.
^ Yeah, $500 should easily be enough for all of them. Of the options, I'd say Vectrex is the best. I'll go ahead and recommend an Omega anyway. It's a few generations newer, but that's about enough money for one.