Bought three used servos over the years (two Komatsu, one Aida). The fault log advice above is gold — but here's what else I'd add from painful experience:
Pull the slide gibs and look at the wear pattern. Uniform wear across the full face = normal. Wear concentrated on one corner = the press was run misaligned, probably overloaded on one side. That kind of abuse stresses the frame in ways you can't see. I walked away from a "great deal" 250T because the left rear gib was worn 0.08mm deeper on the bottom third.
Check the DC bus capacitors. They have a finite life (typically 7-10 years depending on ambient temp). Put a scope on the bus while running at full speed — ripple voltage should be under 3% of nominal. If it's 5%+, you're looking at a $8-15K capacitor bank replacement within a year or two. Factor that into your offer price.
Last thing — run the press empty at max speed for 30 minutes and watch the motor temp rise. Should stabilize under 80°C on the winding (check with thermocouple on the housing, multiply by ~1.3 for winding estimate). If it keeps climbing past that, the motor cooling fan might be degraded or the bearings are going. Motor swap on a servo press is $20-40K installed depending on size.