IM IN THE PROCESS OF DOIN THE SAME TO MY CAR.
although there are a few problems
adding a amp straight to the hi level putputs (sepaker outputs) will create stupid loud and aweful sound! BUT you can solve 2 ways get a amp with high level inputs (not easy to find)
or a converter to change high level into RCA outputs so your normal phono leads.
Now then! outputs on bmw sound system 40watts per channel i hear on the standard head unit.
what i have done is worked out what wires i need to add to wire up a fresh install and try keep the head unit in to keep idrive and add a jvc dvd player and sound system underneath. using audio converter to rca using the idrive for radio and cd and running in the audio in of the new system and through that amp and then adding a 3000watt amp in boot for the bass.
please note you cant get very base'y speakers in the doors at all so dont try buy good slim ones and use your boot for the bass.
So either buy a converter for the amp and use that or try a fresh install which you can put the dvd player or cd player under the normal head unit (stereo)
also if your looking at adding a head unit get the rgb converter and use the monitor as a tv its cool and stops the wife talkin to me as much! BONIS!