TB is right, we have had an 06 KX85 since it was new and it is an awesome bike, it has done a heap of hours both racing and trail riding. My young bloke flogged the wheels off it and it blew up a few times but that has to be expected when a 2 banger has done that many hours all the trouble started when the power valve wore out and hit the piston.
Because they have a power valve they are a lot easier to ride than the YZ85 as they produce a bit more bottom end power which makes them a good little bike for trail riding too.
We also had an 04 that Moose bought off me and he still has it, all 3 of his kids have raced it and it has been flogged even harder than the one we still have and it is still powering.
Couldn’t fault it in the scheme of things, if the price is right buy it.