I owned a dw for 10 years, great machine, never had any issues. Time expander is the main item on these machines. Make sure balls lock and eject properly and that the expander goes up and down smoothly and in a straight manner. Make sure you can trigger video mode by spelling escape. I liked the machine a lot, but it didn't attract much attention beyond me, can be a tough game for a novice.
Make sure topper works, just lights up at times.