I've got the same car - love it. Having said that, I also must state "there is nothing more expensive than a cheap Mercedes."

The M104 motor (other than the head-gasket) is a solid performer. Will last far longer than the 133K you're seeing.

Wiring harnesses are a big issue on these cars. Mine has already been replaced - check to see if this one has been. Good place to look is the wires going to the injectors - they're exposed right at the connectors, see if the insulation looks hearty. If the owner's not looking over your shoulder, take a fingernail and see how tough the insulation is. If the harness is original, you'll know immediately. The main engine harness part is at least $1,000. There's a charging system sub harness that's another $350 or so, plus an A/C (and other stuff like air pump) sub harness that's another $1000. The biggest "failer" in this group is the main harness. It's a DIY project if you're careful, if not, probably 4-6 hours at a local MB-specific indie.

Car is ridiculously complicated, even for an MB. Good thing is that the computer systems are mostly analog - a home-built (or e-bay purchased) tester kit will allow you to diagnose most of the systems yourself.

Also, since it's a 300SE, it won't have ASR (at least mine doesn't) which can be problematic on the 140 cars as they age. It also probably doesn't have other "goodies" which add to prestige, but also deduct from the bank balance - like: rear A/C (the car will have rear A/C ducts, but only an on/off valve for the air, no fan or temp controls), orthopedic lumbar (just a single dial on the door-side trim of the front seats, no buttons with the dial), no reclining rear seats, no up/down rear headrests (just down via a button on the dash).

My biggest gripe about the car is that the 5-speed transmission starts in 2nd gear by default. The car is Waaay to big to be driven this way - it's a real dog off the line. If you rapidly advance the throttle from stop to about 1/2 pedal, it will downshift to 1st. It will also start in 1st if you move the shifter to 2 and then back up to 4 or up to D. My opinion is that the car drives better this way, but I don't bother shifting to 2 at every stop, so I live with the pokiness off the line. Once moving, the car performs far better than you'd expect for 2+ tons with 228 high-rpm horsepower.

I see 22+ MPG at steady 75mph travel. Advance to 80+ (and you can hardly tell the difference between 75 and 100mph), and the mileage drops like a rock to 18 or so. Around town, mixed highway, city returns 17 or so. Car requires premium gas, so a tankfull right now hits around $75 here in Houston. Range is great at about 400 in town, and over 500 on the highway.

Other BIGGIE on these cars is the A/C evaporator. Not all of them fail, but most of them do. When this happens, the entire dash has to come out of the car to remove the evap - AllData shows 17 hours warranty, 23 standard hours. That would be at least $2,000 labor at most independent shops, plus parts.

Climate control is early-generation dual zone. I'm still working the kinks out of mine. Many of the parts are cheap (as defined by no comma present in the price) - several aren't (controller is about $1,000 from the dealer, far less if purchased rebuilt - but still over $500). Having just one component out of whack can cause poor behavior, or lack of function.

If you can do your own wrenching, these cars are affordable and solid transportation. If you pay to have the cars fixed, unless most of the major repairs (harness, evaporator, headgasket) have been completed, I'd stay away. The age of the car indicates that many of the "supporting cast" will be showing their age (window regulators, seat controllers, PSE pump, closing-assist pump, duo-valve & pump). Some of these parts are cheap, but many aren't.

The car I got had not been loved, so I hope that my experience isn't typical. So far, I've pumped about $10,000 into the car (it was VERY cheap to purchase), but I'm seriously picky about my cars. I figure I'd pay $600/700 per month to drive something new/newer, so if I can get a 3 or 4 years out of this car with not much more out-of-pocket, then I'm golden.

Good luck.