I've owned my '91 300SE forever, and can certainly comment on its pros and cons.
- the rear window is very likely the little plastic jaw that costs only a couple of bucks. I have installed 4 now, not super easy, but certainly doable by a DIY'r.
- my car always wears the LF tire on the outside, makes it pull left when the wear gets noticeable - can't be fixed apparantly, I just rotate the tires.
- The 103 engine has 4 flaws, in my opinion:
1.the front timing cover (upper cover) springs a leak routinely. It's about an hour job, heavily documented here. The seal cost is $5 or so.
2. the head gasket goes at about 100k, due to expansion/contraction of the aluminum head on the iron block. I took mine off and carried it to the local shop for a valve job and re-plane - about $300. Again, not really a tough job for a car-handy guy.
3. the valve seals passed oil from the time the car was new - used a quart in 300 miles at highway speeds. The new valve job (and seals) stopped all oil usage.
4. the engine will turn 3500 at 70 mph in this car. This tends to wear the accessories faster. Idler pulleys, water pumps, etc.
5? My timing chain seems good and tight at 180K, but it gets clean oil.

- I complained about the 3-4 (2nd shift) in my tranny from the time it was new - very firm. After 180k, it has gotten even firmer.

Basically a great car, and easy to work on and get parts for. Mine has NEVER
been in a repair shop since the warranty expired, but I have done dozens of repairs to keep it in great shape. If I could not work on it myself, there is NO WAY I would still own it.
That's the way I feel about all older Benz's - if you have to pay someone to work on it, then don't buy it, or plan on nursing it along with a lot of problems for a couple of years before it terminally breaks and you give it away.
Check rear muffler, front suspension/steering, water pump, AC/heat function/vacuum air valves/freon, motor mounts, rust on door bottoms, warm start/idle smoothness, brakes/rotors,driveshaft flex disks, clicking noise in tranny in Reverse, radiator, belt tensioner, water leak at rear of head, egr valve, etc,etc.

DG