I can't help you with the TuV but I am interested to hear how you get on with the S123. Here in the UK, it's hard to find a good one at a decent price - they are all either rotten or restored and expensive. Decent ones may be cheaper in Germany, but remember they have hydraulic rear suspension that is powered from the engine so you'll need to factor in a hydraulic pump in your build. I am not sure but you may be able to convert the rear suspension to normal shocks and coils - I don't think this is possible for the later S124.
These guys (in Holland) have experience with drive trains the TUV can accept:

Drivetrain - New Electric EN
Convert anything you want! Whether it’s a car, a boat, a go-kart or something else you would like to build or convert to electric, New Electric has the perfect solution to repower your project! The possibilities are limited only by your imagination and your drive for a cleaner, better future...
