I'm not aware of anything that can approach its level of power, refinement and reliability for less than 10K.
At the risk of being flamed...I'd say a similar vintage Q45 (92-95) fits the bill. It does not posses the handling or brakes of a 400e but there are fewer mechanical/electrical issues. It is a significantly larger car and taking that into account, it handles quite well. The transmission is both more responsive and smoother shifting (W124/M119 can't even compare). As a result it feels "as if" the Q is significantly quicker - well it definitely is off the line (1st gear is not as tall). I also see the lack of central locking switch be a gross oversight.
The Q doesn't seem to have nearly the electrical gremlins that my 400e does - IE intermittently flashing interior control lights, intermittently working windows.
The Q is more refined as far as NHV and creature comforts. AC is superior both in it's imediate effectiveness, much quiter operation as well as longevity of components not to mention that you actually can choose fan speeds other than just low and high. Ergonomics are also superior. Arm rests, foot rest and accelerator pedal are much more effectively placed (more comfortable) and there is both at tilt as well as telescoping steering wheel that slides back (as does the drivers seat) and up to aid ingress/egress. The seat cushion is significantly longer - I find the W124 seat cushion WAY too short and I'm short!
Paint/clearcoat look as if they are of significantly better quality as well and the paint on my Q, though thoroughly neglected, still cleans up well with a simple wash.
The Q does have it's weaknesses as well: Fuel injectors will go out around 80-100K (around $1500-1800 for all 8), window regulators are weak ($300-400 cost per window - I've replaced three over 110K), upper front suspension links - I've replaced them around 75K). The car is heavy and goes through rotors and tires - rotors last you around 18-25 K depending on how hard you drive. The leather is not nearly as durable as W124 leather and my drivers seat is quite worn (other interior materials are holding up very well). Steering feel is not as good - a bit overboosted for my taste and turn in is not nearly as crisp/quick as my 400e.
Anyhow, just wanted to offer an alternative.