Bought my 2000 4.4i with 105k miles 14 months ago. The transmission issue was known and I gave $1500 for the X5. The following service was performed by myself in the last 14 months:

Full Transmission rebuild. Very rewarding but a full 10/10 difficulty level
Timing chains, guides, water pipe, valley gasket. Also rewarding and 10/10 difficulty level
3.0 rear coil conversion, front and rear bushing/bearing replacement. 8/10 difficulty level
Brake line repair under the driver seat and replacement on rear RUCAs. 4/10 difficulty

I consider these to be all pretty major service items. I'm about $6000 in parts and special tools. This was not exactly "MY PLAN".

It does drive, shift, stop, and handle nice now. I can finally drive the X5 now! However I still have a list of annoying little things left to fix:
previous owner aftermarket radio does not work
passenger window slow operation
sunroof rattles and squeaks in the closed position. Felt tape does not seem to fix this.
windshield and headlight washers do not work (despite replacing the pumps)
rear passenger door lock actuator
air bag light on
O2 sensor code
AC is not exactly cold
rear hatch struts
possible sunroof drain line kink
Needs new tires
fabric on A pillars is loose
recover sunroof shade
passenger door handle broken
and some hail damage and poorly sprayed paint repair.

Maybe I should have started with a cleaner X5. I thought this would be a fun and cheap project. I like the X5 but I could not recommend an E53 to someone who isn't fully committed to servicing and repairing an older BMW. This would be a terrible vehicle for the casual DIY'er who thinks they can do anything with their basic popular mechanics tool set. Good luck getting your wife on board for an E53 project. Mine is begging for a minivan now haha.

Somehow I'm hooked though. I keep perusing FB market place and CL looking for project X5's so that I can justify my special tools purchases.