At 137K, the milage should NOT be a problem. Documentation of proper maintenance i.e. oil changes is paramount in these vehicles. I recommend that, if you are really interested, take it to a MB mechanic for a pre-purchased inspection. It is the best money you can spend in a MB purchase.
A manual transmission is rare. As far as being more reliable than an auto. May be a tech can help you with that.
Valve seals are a big concern with the '86's. They improved the material in later years. A valve seal job is around $300 or you can do it yourself, like some people in this forum have done already. As far as leaks, look in the rear of the engine for head gasket problems. A leak in the front upper cover is not unusual but, most likely is NOT the head gasket.
Door panel skin can be repared or replaced $$. Use that as a barganing tool.
The AC is a BIG problem in these cars. Specifically, the evaporator. If it is leaking, it is at least $1500 to replaced. The only way to find a leak is with dye.
Paperwork. Check that real close. Otherwise, take for an inspection.
