Thanks, Frank. The seller has gotten back to me and confirmed that the evaporator is bad, per his A/C shop. The seller has also confirmed that the car's maintenance records are complete -- I thought it didn't look like that much paper, but apparently there's also a stamped booklet for the first years of ownership.

Since the seller seems honest and the car looks like a good one, I am going to roll the dice on the air conditioning repair and bid on the car. Thanks to everybody for chipping in with opinions. I've been wanting another one of these cars for a long time. I had my grandfather's 300E for a while about ten years ago, but I was 18 years old and could not afford to make the repairs it needed. If I can swing it for the right price, this should scratch that itch.