How should I configure my airsoft setup?

Your choice of weapon is probably the easiest thing to answer, the MP5. I'm not familiar with your specific model or how well it performs, but based on gun type alone that would be your weapon of choice. Reason being is that in a CQB enviroment, weapon size is king, and the speed at which you can manoeuvre and take your weapon from a resting position for a firing position will have a lot of pull in how well you do. SMG's are naturally the best for this while still maintaining magazine capacity and tactical flexibility (M4 is to long, pistols lack ammo capacity), with short assault rifles like the G36 or an AUG with a CQB barrel also being acceptable.

As for whether you should buy a vest or not, my response to that would be to ask whether your site allows full auto or not, and what your trigger discipline is like. If your playing at a semi only site, then frankly I dont see the need for a full vest, a belt with a magazine pouch is enough (in my experience, I only expend 2 midcap's worth of ammo in CQB games at most when at semi only siters, even though I carry 4 in with me). If your planning to play in a full-auto enviroment, then a vest with space for at least 6 magazines + a dump pouch is advisable.

So, extra's, as I said 4 mags is a good number for semi-auto only where as I would advise at least 6 for full auto games. You probably wont need more than one battery (though I like to have two just to be safe) since they tend to last a full play session if you charge them before the game. Also, I would say rather than buying more pistols, buy yourself a good SMG with attachments (sight, torch etc), will serve you better in the long run (honestly, your pistol never gets much of a workout, I never even take my glock in with me unless I want to give my AUG a rest).