The original BMW retro-fit instructions state around 8 hours of work is required, using the full retro-fit kit.

If you obtain all the right pieces, (brackets, bolts, nuts, loom, electronics) you should be able to do it in a weekend.

No modifications - apart from following those in the retro-fit instuctions - are required. You will need to dismantle the interior, run wiring looms, solder wires, and be comfortable working with electronics. The retro-fit instructions help considerably.

Yes, you generally will have to re-program the car so that it knows you have nav fitted. Some people have found it works without, but it depends a lot on the state of the equipment you get. If you purchase 2nd hand nav units that were from a X5, you will probably be OK.

Jochen (e39 with full nav)