Moisture is the enemy to them. Even just what's in the air can corrode the internals of the compressor.
THat's what I'm worried about, and I wasn't thinking of doing the rebuild myself, but thinking maybe rebuilders handled them the way I get a P.S. pump built, that sort of thing.
If I can get a new one for < $250, and rebuilt is around $200, it's not worth the used one.