I've owned several of these and I currently own a 1991 560SEL in very close to mint condition, (It's white and for sale, btw
) and they are VERY good cars. I would go through the car and check out what records he has. If the rear hydro suspension is good and the timing chain and guides have been replaced and it runs and drives very well then it would probably be a good buy.
I do offer a mobile PPI (Pre-Purchase Inspection) in the LA area for a reasonable price for forum members. I can tell you exactly what the car will need now, soon, and in the future and if it's a good car or not. PM me for details.