KWA's are fine out of the box usually. If you really want to upgrade it just so you can rub it in your friends faces, grab some external stuff like a foregrip. Also, tight-bore barrels (6.03 or 6.01) are good to get, high speed/torque motors are nice (They up your ROF basically), grab either a 9.6v battery or if you're responsible enough grab an 11.1v lipo for increased performance, but they're a little higher maintenance, and they're easy to mess up if you don't know what you're doing. It also should come with a KWA "Pmag" which is like a magpul PMAG so I don't see why they wouldn't fit, but I really wouldn't be able to tell you.