250X's are great bikes, and are very depndable if they were maintained correctly.
-It's simple to find out : A leak down test. It wil tell you a whole lot,and is well worth the time and expense to have it performed.
-If the buyer says he ever had the valves shimmed, then you are looking at a valve job very,very soon. Because they never need shimmed until the coating wears off the intake(s), and by then you are very near the end.
- If the seller says he'd ever had the valves replaced (intakes) insist on receipts, contact who did it, or pretend it never happened. If the seats weren't re-cut and valve guides measured then it will be a very short lived top end.Do the leak down.
-If you don't know anywhere to get a leak-down performed by people who know what they are doing
never buy one unless your offer reflects the bike needing a complete valve job, done first rate by pros-(~), a piston, timing chain, gaskets, etc. Done by a pro you'll have a top end good for a
long time, on the order of 200 plus hours of trouble free service, so budget about 750-800 bucks.
-So say the seller says it "books" for 3000 bucks, but its never had a verifiable top end job, or a leak down is not a possibility (unlikely).
If it cant be verified your offer should reflect appx that figure, plus anything else external the bike needs coming off the "book" or you are playing Russian roulette.
There ARE bikes that will pass the test, where the owners claims can be verified though. Just take your time and don't fall for claims of "top end just re-done"

without digging deep. Doug