To power the head unit use the factory wiring - earth, permanent power and ignition source. Forget the extraneous stuff like multiple amps, capacitors etc as this will end with you clawing your eyeballs out. Running your amp and subs is a full no brainer. The stock alternator will keep up with the whole show with an 80 amp output. 8 gauge wire to power the amp (red and black for obvious reasons), an inline fuse holder (weather proof with a 20 amp blade type fuse will do the trick), about 3 metres+ of stereo RCA cable and a roll of blue light gauge wire (this is the remote power trigger wire that runs from the head unit to the amp - without it the subs and amp will play dead). Buy an aftermarket head unit adapter loom to suit the BMW as it will save you time and having to hatchet the wiring under the dash (you may need an adapter for the antenna lead too...I think) Something like this will do the job -
http://www.ebay.com/itm/BMW-Radio-Wiring-Harness-adapter-Factory-to-Aftermarket-stereo-e32-e36-e34-e38-/351160083079?pt=Motors_Car_Truck_Parts_Accessories &fits=Make%3ABMW&hash=item51c2c5a287&vxp=mtr
 Match the colour codes from the aftermarket harness to the Kenwood loom and you can't go wrong. If the battery is already up back then this makes life easier to power the amp but you still need to go through running the RCA's from the back of the head unit along with the blue trigger wire to activate the amp. It is actually a pretty easy deal doing a full install but it is time consuming (I'd allow a full weekend to get all the gear in and have it operational) Be prepared to pull out plastic garnishes and lift carpet to run the connections to the amp - and use cloth tape to secure it down. If it were me I'd run all new wiring to the speakers and solder them directly to the connectors on the replacement speakers but this would do your head in if this isn't your forte.