Just completed the same on my 2002 X5 3.0 d that come with factory cloth manual seats. They are in good condition but I found a complete leather interior (front sport seats with memory and heating, rear seats and all four door panels) some 200 km from where I live for 300 € (VERY cheap) in Montana black. Right rear side bolster was missing, so I'll be hunting for this one.

DAY 0
Cleaned & treated the seats with Zymöl in my living room before installation

DAY 1 (two hours on a Saturday afternoon)
1. Installed door panels, very easy (I've taken my doors apart before to install Dynamat).
2. Rear seat install, quite easy. My replacement leather rear seats were non-heated but I did see that my X5 is pre-wired for rear seat heating (not vital).
3. Front passenger install, easy (I need to chase some a mounting nut threads). The seat electrics are plug and play (as Helmut and others state) at the sole large connector. Changing it without the key in the ignition is enough. The car is indeed pre-wired to accomodate power and heated seats (if you have the heated seat switches in your car).

DAY 2 (less time another afternoon)
5. Next is to install the "magic relay" in the front fuse box before installing the driver's seat. I also installed Osram LED bulbs in the glovebox, under dash & door positions
6. Remove old driver's seat, remove underseat fire extinguisher & rear back pocket panel - transfer to new leather, heated, power seat.
7. Vaccum, find Lego parts, install new seat, plug in everything and test. Doesn't work
8. Make up a jumper wire (Helmuth's method), install, replace blown 30 Amp fuse (see TIPS & LESSONS learned)- all works.
9. Tighten seats, adjust seat, program in memory and rejoice.

TIPS & LESSONS LEARNED:

a) Clean / treat the seats before installation (easier to do)
b) Vacuum your car as you remove seats (outstanding access)
c) Expect to have some small plastic trim parts missing (door panel plugs)
d) You CAN transfer the (front) seat backs with pockets to a seat without pockets (cut the plastic plugs underneath and replace with new, 52 20 1 964 201).
e) Early (up to 4/2002) X5 cars have a different rear seat backrest locking mount than later E53's. I just ordered the early style part to complete the installation.
f) The required relay for the driver's seat is NOT the green relay, part number 61 36 8 373 700. Installing it will blow your power seat fuse and give you an air suspension warning light in your instruments.
g) Use the Helmuth (bridge wire method), which I did or the BLUE relay (I would have, but I didn't have any in my treasure trove)

Bottom line, you CAN change / upgrade your interior, it's easy.

If anyone wants a set of four door panels and front cloth seats (Switzerland) just let me know, cheap to a good home....
I also have a spare under (passenger) seat first aid mount & mount surplus to needs. Fits all E46, E53 and Z4 cars.

If you have questions about "what, how, etc." I know you'll ask