Yes, you could with the G3. From what I've heard, it's accuracy and FPS are very good, I'd just say upgrade to a tightbore barrel (6.03) for even better accuracy.
Not sure about the pistol, seems a little cheap, but that's my opinion.
Those pockets are designed for STANAG-style magazines (aka M4/M16/SCAR/etc). I doubt a G3 mag would fit. Also, I don't think that the vest is very good, I'd recommend a Condor plate carrier w/ pouches, but again, that's my opinion.