Oh man. You definitely need to do more research. I suggest you go on this site and soak in as much knowledge as you can.
If your budget is around the $400 range I wouldn't advise getting into GBBRs until you can possibly spend triple your current budget. WE or WA(western arms) GBB systems are gonna drain your wallet since they can be very unreliable stock.
Still if you really want a gun. Get the WE open bolt system JUST because theyre cheaper (close to your budget), the parts are easily accessible, and they work OOB (to an extent!). That's really it though for WE.
Looking past that, I would go WA or die
. You want something that works OOB and will give you less headaches? Get an inokatsu or a viper tech.
Check out challenger airsoft and see what kind of deals on GBBRs they can give you. Best of luck.