1. Not if it headspaces correctly. If it has good headspace, just shoot it and once in a while check it to see if it gets bad. Until then, there's really no reason to fix it if it's not broke. Make sure the headspace is checked correctly as well.
2. Not at all. If you get one you can shoot (i.e. everything checks out good), you can enjoy the gun while you find and squirrel away the parts you want to use. Find one at a good price is about the only advise here.
3. Not until you have sufficient USA parts in the gun. The non-bayo FS is the ONE evil feature allowed on a US gun. Imports aren't allowed any, so you have to stick with the solid until you get the US parts count correct. Think of the FS on an M14-type as the same thing as a PG on a post-ban AR (i.e. ONE evil feature).
4. Mine did, I woundn't see why others won't.
5. The only thing is cost. Try to get into the gun as cheap as possible. Also try to get your parts together as soon as possible. They are getting harder to find. Mags are also expensive right now.
I own a match grade SA, inc and a Poly that I rebuilt with a new TRW parts kit. The M-1A is pre-ban with all H&R USGI parts. I love the SA,inc. I built up the Poly because I wanted a forged receiver and I wanted the experience of doing it myself. There's really nothing worng with a SA, inc. I'd recommend you do the math first.
Look at what the Poly will cost, what the parts will cost, what the build will cost (i.e. the total cost). Then look at what a used SA, inc gun with mostly GI parts will cost. The SA,inc will be easier to upgrade in the end. Figure some parts and work are going to have to be bought either way if you're going to go with a match-grade gun. Then see what the difference will cost. If the difference in cost is worth it to you, then decide.
I did the Poly because I wanted to do it myself and do it exactly the way I wanted. That meant more to me than the $200 (what the difference would have been to go SA, inc in my case) I would have saved. If I just wanted an M14-type gun without doing any of the work myself, I'd have found a good SA, inc and used it.
The main thing is do what YOU want to do.
Ross