In my opinion, the 300Se is the best all around choice of the 126's. The '88 is far enough along in the life of the model that the bugs were worked out. (Not that there were many).
Things to look for:
1. The transmission, if serviced properly, should be good up to 200K, if not serviced properly,150K. $2500 replace or $1K to re-seal
2. Check to see if valve seals have been replaced/updated. $400
3. Check the timing chain cover for oil leaks. $400
4. Has the A/C been replaced/updated? If not, $2000 lurks ahead.
5. Does the A/C blow out the center vent and not the defroster? If it blows out the defroster, big$$ (labor) or foggy windows in the morning.
Since you may have to pay top dollar, all of these should be verified as being replaced or properly serviced. A nice one can be had here in Texas from $5-8K.
Mine has 260K on it. The tranny was replaced @ 215K valve seals @ 195K. It looks and runs better than almost any new car on the road. These are awsome machines!
91 300E 120K
90 300SE 275K (sold)
92 BMW 525iM 120K
90 BMW 525iA 175K
85 300D 175K (sold)
84 300SD 245K (sold)