Not sure what the prices are for your centre but mine is £15 for the day + lunch + 100 paintballs, but then £7 for additional tubs of 100balls.
Its no fun rationing balls so go nuts

and get several extras!
Getting hit isn't too painful, more of a hard flick with a sting.
However... a shot to any of the following regions is rather painful:
Balls,
Bell,
Head,
Fingers.
The rest of you is pretty much covered in your overalls so not to worry, sticking jeans underneath & a jumper will ease the 'blows' also
Combat tips/advice I would give is, fire off a few shots when you have ample room/time and pay attention to whether your gun has any repetative swerve when you fire- paintballing is far from long distance anyway but getting used to the difference between your aim & hitpoint can be useful when attacking defensive walls/barrells etc
If you see paintballs on the ground- leave them be- most of them will have absorbed all the moisture from the ground and when fired will simply spray a little jet of paint from your barrell- amusing but useless. Also, make sure your generally aware of how much ammo your carrying at all times: there is nothing worse than running to attack a point and emptying out- when you empty out your barrell just emits a clearly visible white gas- clearly drawing attention to you having zero ammo and will no doubt result in you enjoying a moderate peppering.
If young'uns are there on a school trip- use them as decoys- hillarious watching them getting peppered but also excellent collateral damage. If the 'failed TA' types are there, shoot them as much as possible, even when they are walking off/out of play- they take it way too seriously and deserve lots of teeny bruises.
If your event has mounted paint guns on vehicles/turrets- have a go- their great fun!! but at the same time, prepare to be a walking dulux advert in a matter of minutes as they are visible prime targets.

Enjoy pal