If the heavier engine up front does anything (besides drive the rear wheels) it helps keep the front tires from hydroplaning. That is why with tires like the P-Zero, front engined cars use the max grippy Asimetrico on all 4 corners. Mid and rear engined cars use the Asimetrico in the rear but the slightly less grippy (but better wet traction) Directionale up front.
In the 91 model year the 300E's had ASR as an option. Diesels had ASD which is a locking differential. ASR is the more advanced option and is better at reducing rear-wheelspin. ASR uses the rear brakes, throttles back the engine, etc. ASD is a locking diff so if traction is bad at both rear wheels it isn't much help. (twice nothing is still nothing)
You would have to be REALLY trying to break the tail out on a M103 300E if given good tires. OEM sized Michelin MXV4, Energy MXV4, or Energy MXV4 Plus (whatever the latest michelin evolution of that model is) are about the best tires you can buy for the W124's in a touring capacity.
BTW, if/when the W124 V8's spin the rear tires loose it is because they make enormous amounts of torque down low with a huge rush of top end HP. ASR was available on the 400E/E420's and standard on the 500E's.
Having owned a 300E I may be prejudice, but it had a special handling feel that was *almost* as endearing as the rush of thrust from either of my more recent V8's. I believe Automobile magazine said "The 300E is as close to a perfect used car as you'll find on the market today." April 1999. The diesel is a good engine too, but the 300E gives you a more complete package of reliability, handling, performance, economy, and standard features.
Hope this helps...Lee