There aren't really any weaknesses of the 426/450 engines, they're super durable and reliable. I've about 16,000 miles of singletrack trails on my 01 WR426 and it still runs great. I am very faithful with my maintenance, don't race or bounce the rev limiter. The bike may look its age, but it sure doesn't run like a 15 year old bike.
I've posted the mileage at which I had to do work on the WR in the past, but I honestly don't feel like looking it up after a few tall scotches to wash away the day's stress.
Going by memory, a valve guide seal started to leak a little and smoke on startup at about 8k miles, but it was super minor. I replaced the rings, piston and cam chain at the 8k mark after turning the bike into a blue submarine and flooding the engine. We did a full teardown to inspect everything, and I figured why not. Only the cam chain showed minimal wear, but no stiff or seized links. It was not stretched that I could tell, it just had a little more sideways "droop" than a new chain. A single valve guide was replaced at the same time as my valves at around 14,000 miles. My center intake valve wore to the point of the bike not holding compression and not starting. The other valves were starting to show wear as well, but still in spec. Until that time, I had never had to adjust the valves at all, ever. The valve seats did not need to be redone. I switched to the HotCams autodecomp exhaust cam when I did my valves. Bearings are still stock and in good condition.
Good luck with the shopping.
Edited by Pooley, February 16, 2016 - 07:00 PM.