The receivers, barrels, op rods, and gas systems on the Polys are almost always good to excellent. Rear sights and trigger groups parts are a problem on SOME--easily fixed. Bolts are the big deal--usually long headspace and/or soft metal. You might get lucky but should prolly count on swapping the bolt for a USGI and this MUST be done by someone who really knows what he's doing. At today's prices, parts and labor, this will run you $300 or maybe a bit more--depends on how lucky you get on a bolt price.
Polys seem to be changing hands in the $600-650 range lately though I've heard of them going as little as 4 and as much as 8. Keep the bolt conversion price in mind and comparison-shop with used SAs.
Oh, figger on a USGI stock, too, but those are cheap. I've bought several at gunshows for $20 each.