I bought an '87 300E for my daughter and have been pretty happy with it. It is not a new car and I expect and have had some problems. P.O. had used green anti-freeze in it and that seems to take its toll on the plastic in the cooling system. Had to replace the plastic thermostat housing cover and then the radiator when the plastic neck broke. Both were reasonably priced. Cheaper than GM actually! Flushed system with the fancy citric acid crystals and refilled with good MB anti-freeze. Also replaced the temp switch in the head as it had stuck and was keeping the electric fan on all the time. To check plastic, just look and see if it appears new and shiny or old, cracked and degraded.
Voltage regulator died and had to be replaced. That is such an easy job I actually had my daughter do it while I supervised. Took five minutes.
Only other thing I did was tune-up and oil changes. Car runs fair but idles a bit rough. Needs valve seals and will get them fairly soon. You can check for this problem somewhat by watching the exhaust at startup or under acceleration. Mine getting about 1000 miles to quart of oil. Common problem on the 3.0 from what I hear.
If they have the service records, see when (if) the timing chain and rails were replaced. by 150k miles, they need it for sure.
I really hace been happy with this car. Looks good and the level of appointments is quite high. Daughter drives fairly hard and a lot of miles. Car seems to take the abuse well. I like it well enough that I'm going to keep my eyes out for an low priced '88 with manual shift for wife to use as winter driver (she has an SL for nice weather).
Since you are buying from a dealer, you may be able to get him to give you the name and number of the previous owner. Give that person a call and see what they say. Sounds like a good car to me. You'll love it out on the highway