Not uncommon for a tranny of that vintage to need a rebuilt around 150K miles. Lots of good shops and MB's prices are surprisingly reasonable.

No need to flush as you would on other cars because MB provides a drain for the torque converter. Drain pan, drain converter, replace filter, fill and go.

Standard in-radiator cooler. No need to do things differently unless you haul gold ingots or tow.

The thing to get used to in an 88 300E vs 90 Q is the archaic (classic?) look of the 300E gauges and feel of the controls and switches. To the right person, they feel right and shouldn't be any other way. To someone who grew up on Japanese cars (like me) there's a lack of precision, needless heaviness (but maybe necessary to last 500K miles) and downright hostility in things like ACC controls. That's a d@mn big steering wheel, too. If you like it when you drive it, good for you. If you think it'll grow on you over time, it probably won't. This is one of the last cars built by MB engineers. Soon after the accounting department took over and tradition was compromised. The 300E is built perfectly if you fit the mold, but not everyone fits the mold. I sound like I'm a gatekeeper for the club

Sixto
95 S420
87 300SDL