Looks like a NFA lower and possibly a PTAC upper. Figure 400-500 bucks depending on how well you watch the sales. If it works for you and you are happy with it, then there is no way you got burned.
Yep.
The lower is polymer and very light weight. With just irons this would be a very light rifle. However, the light weight come at a cost and the lowers are more fragile than metal ones. When PSA has a sale for $49 or less on forged lowers, I would get one as insurance, just in case if this was my only battle rifle.
I have one and it does fine but I haven't stressed it much and have another to take it's place.