There's nothing wrong with the cast SAI receivers. They've been making them that way for a long time now, including some that were rock and roll, and I've yet to hear of any KBs where the receiver was at fault. A properly cast and heat treated receiver in a semi auto will outlive all of us.
Back to the Polytech, if it hasn't had a USGI bolt put in, it may need it, and that will require a smith who is familiar with the M14 (Warbird Customs, Fulton Armory, etc). The Chinese bolts were soft and the headspacing would change over time. Tack that price onto the rifle if it hasn't had the conversion done.
Get a go/no-go gauge if you decide to buy it with a Chinese bolt in it.
Also, some of the Poly's parts are not to USGI spec (metric threads, pins that are a different size, etc.) Not a biggie, but something to be aware of if you have to replace parts down the road.
Jason