Also depends on what you want to get out of airsoft.
You could just be a more casual airsofter. A basic set of BDU's in really any camo scheme.
An LBV (Load bearing vest)
I'd say 4-7 mags. (For me, it depends on what kind of vest I'm wearing)
If you're wanting more to replicate a specific army and what not, then research about the the type of stuff that interests you. (I have a Waffen SS and Russian VDV loadout)
Don't forget spare batteries!!