My 540, which is for sale BTW... has 257,000 on the clock, and it is still running strong as ever! Plus it'll still smoke most of the cars on the road.
I have not run into any engine problems whatsoever, but then I spend the time and money on maintenance. I posted my maintenance record for the past 2 yrs for potential buyers to review, so you can have an idea what the costs are like.
540 maint record and pix
For an average car, 200+ may be a red flag, but in my experience with my 540, a well maintained 540 is suprisingly solid.