Lots of good info, let's see if I can add anything...
Go to see the car first thing in the morning and hope that it's a cold start. Then, ask the owner to drive for the first few minutes so you can check out the radio, etc. If the owner bags the car while the oil is cold, RUN away. If he/she drives the car gently, allowing the engine to warm, then continue your examination of this car.
Does the owner KNOW anything about the car? When I bought my 190E 2.6 some years ago, I found one where the owner kept going on about the great V-6 engine in his Mercedes. He had no idea of how to operate the ACC and the car was missing the first-aid kit, tools and original spare alloy wheel. Bad signs, very bad signs.
Be very cautious about low mileage. Most cars do not do well sitting unless stored very carefully (lots of Ferrari owners know about storing cars) and they can literally rot away from the inside.
How long has the person owned the car? If less than a year, I'd be pretty wary. Sounds like they're trying to ditch a problem.
Check out (casually) their other car(s). If they're not cared for, but the one for sale is immaculate, smells like a clean up job.
No PPI means no sale. What, you want me to use your mechanic? Don't hold your breath...
1998 C230 "Black Betty"