[GuidoK]

I think that you first need to consider how big you want your brakes. Larger discs and pads (calipers) mean more heat dissipation possibilities, but also mean more unsprung weight.
So lager brakes (foremost: lager discs) mean less handling (grip/traction) but also less heat issues.

The BMW performance kit on the front can be used with a stock setup at the rear if you use a more aggressive pad (according to ECS) for maximum performance (more front bias won't create an unsafe situation, but you just don't get the maximum stopping force out of the whole system).
The BMW (ecs) performance kit is basically a smaller /m front rotor (325mm) and a 6 pot brembo caliper. With some work (for example shims between the caliper carrier) you might be able to fit a 338mm (130i performance rotor) or 345mm rotor (z4m rotor)

The bmw performance rotors are 'semi floating', and rotors like brembo are mostly floating (but there are also non floating front bbk sets, even from brembo). Both systems have an aluminium centerpiece. But I don't think Brembo has a rear GT bbk set for a non /m z4 (and I dont think a lot of other brands have them)