I like the 210 chassis E420 alot, really great driving car.
The surging very well could be the throttle actuator (lots of different names for the same part, so don't be confused. I usually refer to it as the throttle motor). I have replaced a few of these on 119 engines, not unheard of. Not to worry about the harness, that is mainly on about 93 to 95 6 cylinder egines.
On the temp fluctuation, the dual fans are often overlooked on these cars. It actually uses a single motor, the one on the drivers side with the big hub. Try spinning the hub on that side, see if the other fan moves as well, they are connected by a thin belt. You can't see the belt, it is hidden by the fan support bracket, but the 2 fans should turn in unison. You of course will notice if the fan spins freely, not binding. Finally you can idle the engine with the AC on and should eventually see the fans run, if not you may have a bad fan motor or fan control module.