The best advice I can give is that buying a used welder is just like buying a used car. The general appearance of the machine is often a good indication of how it was taken care of. Open the doors, look at the engine, check for signs of major oil leaks (these old engines will often show some small seepage, nothing to be greatly alarmed about). If the engine isn't covered in grease and oil, chances are it's been maintained reasonably well. Start it, run it, give it a workout and see if it stumbles or falls on it's face. The best ones have an all copper wound generator. You will know it by the shape of the generator housing (round). Mine is a '72 and has some aluminum in the windings. Not a problem, you just lose a little heat during the day if you're working it hard. Also see if it has an idler upgrade(older models-after '72 they fixed the idler issue so you didn't have to pin the throttle rod open to use the aux receptacle for grinders, etc.). There are various upgrades for the idler out there, but I prefer the Lincoln O.E.M. version. Maintenance is not a tremendous issue, new spark plugs once a year, change oil every 100 to 150 hours, you're good to go. At some point you will have to replace brushes in the exciter and generator, but they are fairly easy to work on. Someone else may pop in here with some other ideas, but that just about gets the basics.