Run your finger nail across the top and bottom of the barrel where the "smileys" are. They should be either perfectly smooth, or a just barely detectable change in smoothness. There should not be a significant change. If there is, the outside of the barrel has been worn, and will need to be replaced or you will suffer an accuracy loss.
Look at the underneath of the frame rails. You should not see aluminum. Black or dark orange is ok. Many people don't realize that recoil forces are back and UP, and they don't get enough lube on the underneath of the rails to properly protect it from wear. If you see aluminum, pass on it.
You should plan on replacing all the springs on a used gun, since you can't be certain the round count. This is especially true if you plan on carrying it or using it for defense.
http://www.topgunsupply.com for spring kits and other replacement parts for Sigs.
The German P220s are fantastic guns. As long as it was well lubed and not abused, it should be just as accurate as the day it was born. Hope this helps!