I think ChuChu said a lot.
Read the wiki link, make sure all the inputs work, check the screen geometry, check voltages, and screen burn. Ask what's been replaced and when. For MVS specifically, make sure all the slots work and watch it go through the display mode for a few minutes..
Finally, depending on what you know how to do/not know how to do, that imo, is a major factor in deciding what type of cab (condition that is) you can buy. If you have the knowhow to refurb it, then to a great degree condition doesn't matter (except for the monitor which are increasingly difficult to find) but if not, it needs to be in good condition...
Personally, with my first cab I was all about "having a limit" to what I wanted to spend...I regretted that purchase...Since then, I've realized that to get something nice(as my tech/fix it knowledge is extremely low) I'm going to have to pay for it..But at least then you know what to expect and you can really look forward to it, instead of being disappointed with a cab that doesn't live up to expectations...