Yes I know CTCS has them but BLACK is sold out in that link, like I said they sold out fast....
Personally I bought a Gen4 17 for my first pstol, then a I got a gen3 19, and now I prefer the Gen3 over Gen4, if/when I buy another Glock it will be Gen3. All my Gen 4's are converted to Gen3 internals anyways, and I dont use the back straps and I prefer the mag release on the Gen3 also...
As for the poly frames I have not seen any %80 GLock frames that have metal inserts for the frame rail slides like an OEM GLock frame. People always say Glock are all plastic, but they actually have metal molded into the plastic for the frame rails to slide on... %80 frames you have to cut those rail slides out of plastic with a little cutter they supply in the kit, so it remains bare plastic after you make the modifications to take it from %80 to %100 functional
My Glocks show sings of wear on those metal inserts (like pictured below), so I really question how long those will last with metal on plastic sliding contact.
It seems the Polymer 80 kit give you a custom locking block with forward rails attached to it instead of the frame, thats better then no metal but again its not OEM design and still has no metal in the rear rail slides. $300 is not cheap for a lower, so Id hope it would lasts as long as an OEM glock lower. I paid $160 US for OEM bare Glock lowers for a 17 and 19 so its kinda steep, although it includes the jig and bits that you would only use once