IMO, with the 4.6, and with most BMWs, maintenance history is everything. I would certainly try to find a truck with the big things replaced: transmission, cooling system, and chain guides.

Buy private party from an enthusiast. Spend a little more at the beginning for a nice example. I don't think the 4.6 is selling very well right now. Gas prices, and plenty of "regular" X5s on the market are to blame. I would think owners needing to get out of their 4.6is' are willing to deal.

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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