Hello steven, I too want to do an oem upgrade.
what you need is a lot of stuff depending on the options your X has already.
if you tell us we can show you what to elimintate
I just dont like the look of anything aftermarket for the X.
1. retrofit harness <- easier or OEM harness<- harder
2. nav computer dvd/cd drive - Mk1 - 4 <- 4 is the newest for e53
3. navscreen and tapedeck behind it<-some reason some people just sell the screen
4. New radio-mounts in rear of vehicle - where X is prone to flood
5. gps antenna
6. Nav cd/dvd - online free to about 100.00.
7. hardware mounting brackets for rear left of vehicle
8. a dremel or saw
9. install guide, time, and patience
10 this site since a few members already did it and wrote walk throughs
11. complete retrofit kit in the US is almost 3000 bucks -
BMW X5 2000-2006 E53 BMW X5 E53 Navigation COMPLETE Retrofit Kit although I have found cheaper parts on UK ebay.
12. the abillity to rip your X apart and put it back to together again.lol