i retrofitted my e46 with navigation and tv - basic you need a retrofit cable for 16:9 system, 16:9 display, computer navigation color mode coded(choose mk4, mk3 is much slower and have some limitations...), radio module (bm53 is latest used on us, try to get one mfg after 09.05 - have text capability or at least after 2003 to have aux in) - that's the minimum required to have navigation, you can add tv module (can be done later but need extra cable between tv module and navigation computer), also can add tel module (lot of options, choose a later bluetooth tel module to have better compatibility with phones - w/wo ses) and a looot of work witch can be done by someone with knows what is doing, you can easy fry something if misswiring navigation loom to car harness...is not like a rocket launch to the moon but is not an easy task, trust me

p.s. -you can use jochen's navcoder software to have mk4 coded, tv and a lot more - very useful software for coding some options
p.s.2 -i assume you have a e53 x5...