LS (your choice on which variant Chevy engine) engine with RWD setup. You'd have all the power you could imagine but cost, depending on which LS you went with, and effort would be interesting.

Forget the pano roof idea unless you just HAVE to have it. Then no amount of reasoning would talk you out of it. And if that's the case, it might be worth it for your personal enjoyment but I don't think anyone here is going to recommend that.

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2005 X5 4.4i Build 04/05 Maintenance/Build Log
Nav, Pano, Sport (Purchased 06/14 w/ 109,000 miles) (Sold 8/15 w/121,000 miles)

2006 X5 4.8is Build 11/05 Maintenance/Build Log
Nav, DSP, Pano, Running Boards, OEM Tow Hitch, Cold Weather Pckg (Purchased 08/15 w/ 90,500 miles)

2010 X5 35d Build 02/10
Nav, HiFi, 6 DVD, Sports Pckg, Cold Weather Pckg, HUD, CAS, Running Boards, Leather Dash, PDC, Pano (Purchased 03/17 w/ 136,120 miles)