As stated before, Brabus is an aftermarket company like Renntech and Kleeman, AGM is pretty much factory. Brabus makes great products, I have owned a fully built Brabus Benz before and can say that they are a very high quality tuner, but they carry a hefty price tag. One single rim/tire combo cost nearly $2k once! Usually unless you just get a Benz with a few Brabus upgrades, you can buy a fully built vehicle. They typically increase displacement quite a bit with their naturally aspirated vehicles, but if you are upgrading or buying a factory forced induction Mercedes then I would suggest checking out Kleeman and Renntech products as well.
If you have any specific questions feel free to shoot me a PM.