Since you've owned it since 85K (at least) to 270K you sound you know that despite the high miles it is in overall good shape. That's a plus. I vote to look around for a good used transmission especially since you have access to a loaner vehicle. I guess $3,200 doesn't sound too awful for a rebuild. I've seen quotes as high as $6,500 for the GM trans. Yikes. I'm not sure how many miles a year you drive but say having another 2-3 years of driving until you hit 300K would help you come out ahead even with the rebuilt option. The M54 should last until then as long as it doesn't overheat.

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2001 3.0i * SOLD *
Current
2010 E83 Alpine White