I'm not an expert about M14s in general or Polytechs in particular, but I do own a Poly and I've done a bit of research about them. So, while taking what I have to say with the appropriate grain of salt, here is what I have read and experienced from the US-legal Polys (which are the earlier ones, since importation was banned awhile back):
The Polytech M14S is a great example of the utter stupidity of communism. They went to the bother (and, I would imagine, significant expense) of manufacturing an outstanding receiver, properly dimensioned in every respect, forged of tool steel with proper heat treatment. Basically, it is every bit as good as a real-deal M14 receiver, and arguably better than the cast commercial offerings most of us own today. They then mated it to a decent chrome-lined barrel. They then slapped on some parts of highly variable quality, installed a poorly-dimensioned bolt of too-soft steel, finished everything like it was a homework project from Parkerizing 101, and slapped it all into some mystery-wood stock. Naturally, the Polytech rifle got a reputation for shoody workmanship, especially when things started exploding near shooters' faces. Bad news, eh?
Not so fast. That receiver is excellent. The barrel is good. The rest of the parts can be swapped out as needed. The bolt should be replaced, or at least watched like a HAWK with headspace gauges. The stock should probably be swapped out. But once you've done that, you have one heck of a rifle. The receiver should outlive your grandkids.
If you have a line on a M14S for under a grand, I would give it a good, hard look. You have to factor in the cost of buying and installing a USGI bolt (under $400 total, I would imagine) and expect to do a little fiddling to get stuff up to snuff. Or, see if that work has already been done. But once you do that, you'll have an outstanding rifle, for way less money than you would pay to get the equivalent from anyplace else. And, since importation is banned, you're not supporting communist slave labor by buying it.