I have been riding for 25 years now
I race +25 B and Vet B in MN as well as doing the racer X series this year, had a pretty bad injury last year
I currently race a CRF 450, see attached pic
If you are under a budget, get a 2 stroke, you will get a newer, nicer bike for the money
As for 4 stroke dependability, dont buy too old a of a 4 stroke, 05 and newer would be best
The valve issue is more of an issue with a bike that is not properly maintained, this BS you hear is just that.......BS
It is hard to tell if a bike has been well taken care of beceause they can be prettied up easily but here is what I would look for
1. DO NOT BUY A BIKE THAT SAYS NEVER RACED
2. Look at that chain/ sprockets
3. Dont worry about tires, they go to crap after 5 rides anyways
4. Check for the fork and shock seals leaking
5. Does it start easily
6 how does thet clutch feel, if it is grabby it may need a new basket
I would personally never buy someones trail bike, they are beat and never get the needed maintainence. I have found that bikes raced by A and B riders are well maintained.
That is my two cents
What state do you live in?
try to find the AMA district web page for that state, for MN is is ARMCA.org