There is an entire forum dedicated to the w211 E55 over at mbworld; suggest you go browse there. It has all been discussed a million times.

To answer your questions: the car uses an open diff. The traction control can apply each rear brake individually to manage power delivery. There are aftermarket limited slip options.

Not a fan of modified engines. You can only add a bit more boost before beginning to run out of fuel. Cylinder #8 goes lean and then blows up. The HP claim in the advert is bogus, nobody is getting that kind of power.

Service is every 10K miles or once per year. The car tracks time and miles and tells you what is needed. Transmission service is every 40K. Brake fluid at 2 years. Coolant is 3 years, I think. Belts every 40k miles. Mtce is not too bad.

Learn about the more common problems and cost to fix them. It's a great car but can be expensive to repair.