to begin with i wouldnt reccomend WE they dont have the best metal, also they have a lot of problems with leaky mags and stuff like the slide not locking and in general just not the best for beginners,what i would reccomend is a KWA usp or kwa m9 because kwa has probaly the best gas pistols atm, but if you were to get the WE then it would do about 2-3, 3 second burst of gas
ok, the mags are normally filled from the bottom via a golden colored brass inlet valve. Now, some people say store them with gas, but i don't. I follow the other trend of storing them empty so i know the seals inside aren't doused with propane among other things.
When you get your new replica, please try to get a tech to remove the discharge valve magazine and lubricate it with 100% silicone oil, get a generous amount around it then re-install it. this will keep the o-rings well coated. try to do this roughly every 10-15 refills. this is will prolong the life of the o-ring seals. Also for the inlet valve, just drop a tad of silicone lube then inject some gas so it travels inside as well. That's pretty much it