Not a huge fan of G&P. take that 500$ and buy a nice JG that you think looks nice, put some upgrades into it, and spend the rest on other things you will need, trust me, the gun is a small part of the cost of airsoft. And the ARES quick spring change is pretty cool. You can get an M4, put this in it (not from this site, couldnt find many other links)
http://www.evike.com/product_info.php?products_id=33763
You'll have spare parts in your stock gearbox, and if the poprietary parts break, just pop the other gearbox. Don't be afraid to buy from well known chinese clones, they usually make pretty solid guns.