Classics! The classics market is almost dead right now. Those games from the 70s and 80s are still a blast to play. And you can find lots of them for less than a grand. I know a lot of people don't find them as exciting as a brand new DMD game, but they can be just as much fun. And the early SS machines are pretty easy to work on. So easy to move, easy to work on, and a hoot to play. Sign me up for a Mata Hari! (oh, too late, I own one).
Of course, for that money, they will not be collector quality. They will be a good honest game. And they will have signs of age, but you can play them hundreds of times and still just be getting to know them. Show your mad skills on machines that were built to just take your quarter!