I bought a used 98 WR400 a couple months ago for $1675 and had to put plates, springs, left side radiator, front & rear sprockets and rear tire on it when I picked it up. I think a 2000 should go for around 2K so maybe you are right to be cautious. Sometimes you get a break though.
I agree with Dan. See how easy it starts and go through the gears. This is my first 4 and noticed the valves are a a little more noisey than I am used to. Maybe this is why you mentioned the valves? My buddy has a a 2001 yz426 and it sounds the same. No smoke, no joke. Just bang gears!