A "should I buy" question

[JimZ]

I'm looking at an '85 300D Turbo Diesel, W123. The car is for sale at a strict Benz repair shop. The guy only fixes these cars, only uses OEM parts for everything, and he's been working on only these cars for 25 years. He gets them into the condition where the only maintenance the buyer will need is oil and filter changes. He makes everything as close to perfect as he can.
Having said that, the car is going for $7500 and it's pushing 300K. It is my understanding that if the car is overhauled to the point that this one is, the milage doesn't really matter because the engine will last forever. So my question to you is, is this a good deal, and how many miles would make you shy away from purchasing a car maintained to this level?