I don't know what PWA is....what their level of quality, etc is....never heard of em....what I can tell you is my friend bought a new Olympic arms K3B for about $550. But the internal parts and barrel were of much rougher machining than my CMMG, which cost about $1000 for the basic gun.
I concur with the fellas who said that you will probably end up spending about the amount you spent on the gun to "upgrade it." But the parts probably will be an imperfect match, and you will probably end up feeling disappointed. Plus, my own personal view is that I would never buy a used gun, especially one where you don't know how many rounds went through it, you could end up having to replayce the barrel, firing pin, who knows.....there's nothing like having a brand new, only test fired ar in your hands, all shiny with the factory grease still on the bolt smelling wonderful!
For what it's worth, my personal philosophy is to NOT go low end when you are buying a gun. My friend has gotten burned using the "quantity over quality" strategy....he bought a Llama 1911 clone....it was $200, rock bottom for a 1911, and he got what he paid for, the metal felt like wrought iron, and the slide retaining lug broke off after about 100 rounds.... If you are looking to buy an Ak, get a GOOD top of the line AK. If you are going to buy an AR, get a good AR. Colt, LMT, CMMG. There may not be much of a functional/reliablity difference between RRA and CMMG/LMT, but the differences in finish and precision of the parts is quite noticeable. So if that's important to you......ya know....
My 2 cents.