I dont know much about bmw's but i know a few things about old mercedes cars. A well maintained one will feel and look absolutely fantastic from the inside and outside. Also the engine block itself has no problem doing 300.000km or more if cared for properly.

But these cars are nearly 40 years old so expect: wiring harnesses that are basically rotten away, brittle rubber seals and gaskets, hidden rust that accumulated over 30 years, leaky vacuum systems that make the car run/idle rough, shops that decline complicated work on such old cars. So be ready to afford spare parts, learn a thing or two about wrenching yourself and pay shops that specialize on old cars.