A WE G36 is a good starter gun.
However, make sure it has a NPAS fitted as it can run super hot. Also, the mags are much thicker than standard M4 type mages so you would need to buy bigger pouches to fit them in.
When I use GBB's in game, I normally take six mags in with me, which means the extra expense of buying them. I suppose you can get away with four mags if you are conservative with shots and have cover to hide behind when you are reloading. Then there is the gas, you will needs a few different power types depending on the time of year you play. So it all adds up.
BUT, the most important thing with GBB's is that they are super fun and once you start playing with them, you won't want to ever go back to using AEG's.