The most "bang for the buck" will be the mid '80s MCIs.
MCI parts are cheap. If you can do all the labor yourself, most anything can be fixed, rebuilt, replaced, etc. without breaking the bank. Labor is what gets you. Some of these repair projects can be time consuming even for a professional bus mechanic. If you have to pay him by the hour, get the checkbook warmed up. I've done everything on the bus myself right in the driveway. I did kingpins and bushings, brakes, shocks, airbags (inspected and replaced as necessary), leveling valves, dryer rebuild...I basically went through the chassis completely and replaced what looked like it needed replacing. If I had to pay someone to do that, it would have cost a small fortune.