That's a lot of scratch to put into an 8-9 year old car, although it is a known problem with the 4.8 motor, I would just go ahead and get the X5M, as long as it's a low mile one still under some type of warranty. I'm looking at buying one soon also, my 4.6 is long in the tooth and I'm feeling the need to upgrade, I need more horsepower. ..lol!

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2002 X5 4.6is
1999 C5 Corvette
1992 Mitsubishi 3000GT VR-4
1994 Kawasaki ZX-9R
2005 Mazda 6 Wagon (Wife's)
1991 Mitsubishi 3000GT VR-4