Knock sensor is a PITA to replace. The whole intake has to come off.
But 7500 seems like a pretty fair price if it doesn't have too many issues. I paid 16k for my 01 suburban back in 06. Only issues I have had besides normal stuff like battery, belts and brakes, is my waterpump went out about 6 months ago and now my AC compressor went out. Other than that its been very reliable for the past 5 years.
Ehh, it isn't that bad. I've put a few on Corvettes and the trucks are easier.