one thing to look for is play on the connecting rod
(i realize you cant do this untill you get the bike home and the cylinder tore off but you sounded convinced you'r buying it allready)
so, with the cylinder off you can check to see if there is any play on the connecting rod
it is OK of it slides left to right but if there is vertical slop (up and down) your in trouble
to give you an idea what you might be looking at, here is the build of my project bike
i paid 400 for the bike and total investment right now is just shy off 1100
thats with a overbore, replate and over wiseco top end kit
http://kdxrider.net/forums/viewtopic.php?f=77&t=12547