a GBB pistol and as many mags as you can afford would be my preference. The TM GBB MP7 is fantastic too, but a pistol makes a good secondary if you ever venture into a woodland site. If you play woodland then a CQB M4 would be good for both worlds and would allow you to use hi-caps which helps keep start out costs down. I'd recommend in that case the PTS RM4 ERG, but this all depends on your budget?
If you go for a pistol, I'd avoid ones with full auto, they are very fun but empty mags way too fast, a TM G17 is all the pistol anyone needs, but any TM GBB will do you right in CQB, if you play woodland though range is going to be an issue.