From an expense standpoint, the 4.8 shouldn't be all that much more expensive to maintain.

The only really expensive difference would be the engine. If that goes, it's hard to find another 4.8 to replace it (compared to a 4.4).

The air suspension on all 4 corners is another difference that is more costly than the 3.0 or 4.4.

Otherwise the rest of the components will have a similar expense to the lower models. Of course the up front cost of buying the 4.8 is more.

It sounds like you have the scratch, go for the 4.8...

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2002 X5 4.6is - Titanium Silver / Black Alcantara - Dinan Stage 2 - Bilstein/Eibach - Brembo BBK ----SOLD---
1993 318i - Brilliantrot / Tan - bone stock daily driver