The 400E W124 is not exactly thick on the ground in the USA, I don't know how many were imported into the US from '92 to '95 but there weren't too many.
A Remus exhaust might be a nice addition, for about $700 I'd guess to install, and give you about 2-5 hp, it would look and sound pretty nice, and not drone at 70 mph.
I love the W124 400E, they are balanced even better than the 500E, and WAY faster than the 300E, and tons of the 300E were sold so you'll always find parts in the wreckers yard for them, and the 400E was just as dead reliable as the 300E, which except for wiring harnesses on some of them, was about as bulletproof as a modern Mercedes could get.
I've tested out a W124 500E, scary fast, you could practically slice and dice freeway traffic with supreme confidence at 115 mph, as I did in San Rafael, Calif. years ago, the brakes and cornering were supercar fast
www.500ecstacy.com and the 400e felt to me like a detuned version of that car, balanced even better, and perfect for really quick day to day everyday use.
For a damn near perfect daily driver, the anvil reliable 300E with a huge shot of adrenaline, the 400E wins HANS down. Super Mercedes comfort, reliability, durability, safety, last E class with the old world MB quality, ("The best or nothing") Gottlieb Daimler's slogan, and with a stealth extra.
Steering wheel a bit too large for my liking, and the pedal throw is too long )like on the 560SEC) but stomp the go pedal to the floor on the 400E, and it'll go.
If you have extra performance money to spend on your 400E, spend it on a set of Euro lights, if you don't have them. The appearance is awesome. First ones I ever saw were on a W124 220CE coupe in Lima Peru and I was floored. They look stunning.
Plu8s, when they are installed and aimed you will love the way they thrown a beam pattern at night, and are much brighter than the stock US DOT units.
A no brainer. About $500 new from Capitol West, etc. and less on ebay Germany.
You'll get the US plugs for the lights, it's an easy install. You will love the result. For "OFF USE ONLY" What a (bad) joke. Stupid US DOT lights.
But I've never heard of any policeman of CHP officer writing up a violation for Euro lights on a Mercedes. Ever. Probably the typical LEO wouldn't know the diffrerence anyhow, and had better things to do anyway (hopefully).
If you STILL had $$$ to spend, how about a Hella or Fiamme airhorn with a compressor. That would be cool.