I saw the back plate off of one a few years ago and it looked pretty stout inside, with a proper set of rods and insert bearings. I wouldn't hesitate to buy one if the price was right. Probably a damn site better quality than anything you are going to get new for the price. Who knows, Schulz might even still support these. If it did fail, you still have a top quality 1ph motor and tank to work with. If it runs ok and doesn't blow the interstage safety valve, I'd say it was a good bet. Performance should be on par with any 5hp compressor unit, since there are no real secrets to making air, just different designs based on cost.
Check to see if the interstage safety valve is the proper rating if possible. If it's 200psi, it's too high and can mask valve problems. See if you can drain some condensate out of the tank. If it looks like gravy, take a pass.