Both the M103 and the 104 are pretty mechanically bulletproof as long as they are maintained properly. The 104 is an interference engine (don't know about the 103). Any car you consider at this age/miles will need repairs sooner or later. The head will need to be gone through (at least for valve stem seals and guides) because the seals are shot by now. Budget $1200 for a head rebuild if you R&R the head yourself. If the car is using oil/marginally passes smog due to HC, you know the seals are gone and possibly the guides. Experienced guys will tell you the 104 head doesn't use up guides as often as the 103 but if you have the head off you might as well do the guides. The 104 is a DOHC 24-valve motor with variable valve timing that makes quite a bit more mustard than the 103. When it hits about 4000 rpm it really wakes up, sticks you back in the seat and makes this terrific sound as it revs.

The later 124s came equipped with 134 as the others have said here. Mine ('93) blows ice cold.

There are a few sensors but they are pretty reliable. You don't read about too many sensor problems. I don't think any of them are outrageously expensive. A car of this age might need new cooling system sensors. The bottom ends on these motors are way stout and they will run over 500,000 if they have been maintained. The heads will have to be rebuilt every 150k or so. I had my 104 head off this summer for a rebuild and with 145k on mine, there was zero ridge at the top of the cylinders and the cross-hatching in the bores is still plainly visible.

The biggies on these cars are the wiring harness, the evaporator, and the head gasket. Some cars need the harness and some don't. Mine doesn't - I got lucky and believe me I have checked. Same with the evaporator: some need it, some don't. I read someplace that we 6-cylinder 124 owners should just plan on a head gasket replacement and timing cover reseal being on a 5-year PM cycle because they ALWAYS leak eventually due to the design. You should plan on replacing all the components of the cooling system (radiator, pump, all hoses, fan clutch, t-stat). Budget $650 or so for that if you do all the labor (some of the hoses are $25 - $30 each). If you get an ASR car or a 4Matic car, you better have a big checkbook. There is a reason why 4Matic is referred to as $Matic. Another thing. A car of this age will need a suspension rebuild because the rubber bushings are getting hard by now. So you should plan on that. But that is true with any old MB. The transmissions are spotty on service life. Mine has 142k and is starting to do the dreaded reverse-delay which means it's on its last legs. Some guys have gotten over 200k out of these trannies. I think the PO of my car did not R&R the trans fluid and filter like he should have and I am paying the price.

Take any propose car to a MB indy wrench and get a pre-purchase inspection. This is imperative. Remember the old adage: There Is No Such Thing As A Cheap Mercedes. I learned this the hard way. And recite the MB Mantra: fluids and filters, fluids and filters, fluids and filters. :-)

Hope this helps! Pete