Where to start, first off, check the repair records of you 500SEL closely. If it hasnt already had the evaporator coil replaced, get ready, it will need one soon and at a cost of $3000-$3500.
Water pumps are weak, and the upgrade is recommended as my 1992 500SEL ran hot most of the time.
Power window swithes were replaced 4 times
Batteries were replaced 3 or 4 times I forget,
The 1992-93's eat front tires, three sets in 3 years (Mercedes paid for them, all over $600 a set).
There updated ball joints the really help with the constant "shimmy" I experienced with my '92. $500
I had some window regulator box in the trunk replaced, it senses the pressure on the windows as they go up, not allowing them to go up fully. $500
I had the entire guage cluster replaced (Hint: Dont have the key turned on if you jump the car to get is started, it will blow out the dash) $1500.00
Transmission needed constant adjustment, and linkage replaced. Also, could never get rid of the "rolling" after it was put into park, was told the 12" of "roll" was with-in spec's. Funny, my 1996 doesnt do that.
Trunk pull down and keyless lock didnt always work. Replaced the entire central locking system $4,000 (apparently you cant just replace one part, they whole system has to be updated)
Oil leaks, had several early on, but they seemed to have corrected them.
Some sensor that regulated the seat heaters wsa replaced, as whenever you started the car they would come on full blast.
A/C Fan switch replaced.
Some rubber piece on the throttle broke while driving on the freeway, making the accelerator innoperable, That was real fun!
Stereo Head unit replaced, dolby switch didnt work. $1000
Sometimes the reverse guides would retract sometimes they would not, never had anything done, as they always worked at the dealership.
This was all UNDER 3 years and 35,000 miles....SCARED YET?????
The last year I owned was relatively trouble free, but
Dont know if you can get a warranty with a car over 70,000 miles, at least not a warrentech.
Hopefully mine was the exception, but from what I have heard it was the norm.