dirtfreak,
I have to agree with Rcannon. If your bike is used, who knows how the previous owner cared for it. Especially if he was loosing interest in the bike, I'm sure he lost interest in maintaining it too.
I also bought a used bike, an RM-250. The bike was dirty, and the air filter was even dirtier. I took the pipe off and inspected the cylinder. Using a flashlight I observed the piston while cranking the motor. It's alot easier if you remove the plug. My piston was very scored. I reached in with my fingers and could physically move the piston in the bore. When I took apart the top end, I found the plateing was totallly gone around the exhaust port.
I posted the same question as you are, and was dircted to:
http://www.ericgorr.com/
I contacted Eric Gorr at Forward Motion. It's best to call early in the morning. He even answers his own phone. He can give you good advice. I wanted my bike to be mechanically sound, but wasn't going to race it. I didn't need the super polish and port job. He saved me some money by swaping my head for a servicable one. (within factory specs) He sent the cylinder/piston/gasket kit for around $200 after crediting me for my cylinder. He also suggested installing a new wrist pin bearing, which I did.
He's a good guy and a supported vender. Give him a call and see what he thinks. If your gonna keep the bike for awhile, you might as well make it right.
Good luck, Steve.