Congratualtions! That is great that you are getting your E53!
As for the cam, it can definatly been done. I think you need the towing harness to wire the cam to the car. The GPS system can be updated with a new disc i beleive. Do a search.
As for the rims, if you put the 26" dubs on your E53, you will be banned from this board.
In all seriousness, I don't think 26" will fit. Do a search and see if other members put WAY oversized rims on their E53.