Bigger might be better.

In US trim:

M116 (420SEL) 200hp
M117 (560SEL) 240hp
M100 (6.9) 250hp
M119 (500SL) 320hp

I doubt the cost/performance ratio of turbocharging your 420SEL will be better than just selling your car and buying a 560SEL... whether or not you consider resale value. An M119 would be an absolute steal based on the same ratio.

More questions for turbocharging:

- how will you get more fuel to match the airflow?
- what's the capacity of the M116 system?
- is it just a matter of installing a bigger fuel pump and FPR?
- will an M116 idle with an M117 fuel system?

- how long will the transmission last?

- will W116 or R107 exhaust manifolds fit a 420SEL?
- some early 70s engines have cats in the engine bay, might be a good
spot for turbo/s

This is fun. I'm sure a turbocharged 420 would be even more fun.

Sixto
91 300SE
81 300SD