How many miles does the 300 hrs equate to, was it alot of street, back roads gravel, or all trail and/or actual racing. I'd want to see the receipts for the top end work and maybe talk to the dealer that did the work. I've owned three EXC450's and currently have an '07 with 94 hours and 2678 miles. They have all been solid. I know a guy that recently sold an '07 525 that had over 14,000 miles on it and a fair amout of them were pretty hard riding with sport bike guys on the street. The motor had never been touched with the exception of regular maintenance, (and I'm not certain how much of that it actually got). I can't help but think that bike probably needed a head job and at least a re-ring and piston but you couldn't tell it by the way it ran. The motor was quiet and didnt smoke either. The countershaft leaked a little bit but that was it.
'00 FXDX SG Sport, '06 300XCW,
'07 EXC450, '01 640 Lc4E