Jeff,
You're smart to ask questions. Here's my $.02. Barrels are the most important. The rib should be solid, tubes un-cut and un-altered in anyway with no significant dents. You can support the barrels by the hook on your finger then tap up and down each barrel with a screwdriver handle. The barrels should "ring" similar to a bell. If they buzz/rattle or thud the rib could be loose or it could be the extractor rattling (not a problem). Now, a ring doesn't mean that the rib is perfect but it is a good indication. Next the gun should function correctly. If it is tight (remove the forend and shake the gun feeling for play) that is preferred. However, given the work that's going to be done a "little" play can be addressed. If you're going to try to use the existing stock no visible cracks, chips, severe dents would be best. Not to down play internal cracks but these can be addressed without affecting the external aesthetics.
Good luck and keep asking questions.
Phil
