Something to consider if your buying a shooter is the serial #. Below 800,000 for Springfield and 285,507 for Rock Island are "low numbered" and considered unsafe to shoot(all Rem 03's are OK). Had problems with the heat treatment early on. Many people shoot these and many receivers are just fine, but there is a risk.
Lots of "high numbered" ones out there if your just looking for a decent rifle. Your average rifle will have gone through a rebuild and have newer barrel and other parts replaced (bands, bolt, stock, rear sight). That same rifle with a low serial # does not have the same value as a high numbered one.
Like Chuck said, make sure the barrel is good. Check the muzzle and look down the bore. Headspace is usually not a problem, but it doesn't hurt to check or ask. Make sure the stock does not have any cracks, especially behind the tang. Grasping grooves would be a plus. Milled parts vs. stamped are a plus. These things are cosmetic, but are correct for 03's and effect value.
Your average shooter will be $500-700, again depending on small parts. "Correct" rifles vary widely.
Decent 03A3's are in the same price range.
Buying a good book can help alot, if you can stand the wait. Post pics before or after you buy, if possible. Good luck!