As with any car, don't buy it if you could not afford to repair it. I could get a replacement engine for my common C220CDI for 1500 quid, which I could afford, but I wouldn't want to have to stump up 7k for a C63 engine. That said, it is true that you wouldn't even buy a Lexus if you just went on internet reviews. What is the real percentage of failures of this particular engine? If you really like the car and are prepared to spend for peace of mind, get the bolts done anyway. Caveat emptor and all that.