Heater does not work can mean more than one thing.
does it blow air but not hot? does blower motor not work? without knowing EXACTLY what you are trying to say it could be anything from a $2 fuse to a blower motor that is toast. Add a little clarity and this forum can provide you advise.

I have heard those motors are pretty great but I do have a friend that had to repair a head gasket on the same car you are looking at.

 

A FREQUENT complaint about the heating system is that air does not come out of the center vents (above the radio and environmental controls). This is NOT a problem...it's the way the car is designed. If you call for cold air in the car those vents should open then close off again when heat is called for.
As stated by the previous poster, a little more specific information will get you the information you need.

 

Another common issue that you should investigate is to look at the condition of the engine wiring harness, particular at the connectors. If the wire insulation is flaking/cracking or is missing altogether you will need to buy a new engine wiring harness. The diesels are least prone to this, but not exempt.

All MB models from 92-97 had this issue.

 

Mine is bullet proof but the main things have been done. Head gasket has been replaced with the updated version, Vacuum pump has been replaced with the updated version. A/c evaporators go out on all years, $2,500!

Get a Pre Purchase Inspection from someone who KNOWS these cars.