I’m looking at a 2007 KW with a CAT C15 475 HP, is the C15 a good motor?

Have you driven this truck? Do you know which exhaust manifold and turbo was installed? If this thing is not tuned properly it may be a turd in the lower RPM range.

 

Really need more info. Has it been overhauled? Are the IVAS still active?

 

I totally agree. You need some in-depth info about this motor. What manifold and turbo? Which tuner? Who wrote the tune? Is the tune locked or password protected? When the truck was tuned was it verified on the dyno or just road testing? And also, are the IVA's still being used?

 

Yeah without that Info I wouldn't buy it unless the price was really good.

If the IVAs are deleted and it's still has 18:1 compression pistons it would be prone to blowing headgasket unless driven carefully, but also depends on the tune.
If its just been single turbo swapped and IVAs are still active then it's probably fine. But seems most delete IVAs when they go single.
It can be a really great engine, just need more info to know where you are starting from

 

What he said
Might blow a gasket

 

What does it look like on a dyno?

what’s the blowby?