Chest rig is just to carry mags and whatnot. It can act as "protection" from welts but mainly to carry crap in for easy access (mags, radio, med kit, killrag, etc.)
Pretty much like this
except I think ASG's ones are Olive Drab (Green) because that's the dominant colour nearby the coast (while people like me will use Coyote Brown in badlands and Prairies).
BTW I think he needs 24 hours notice to prep a rental package for you (so he knows how much stuff to bring to the field) so if you do it by now-ish you should be all set to play when they have their weekend game.
As for gameplay I'm not 100% sure but usually 10:00 AM muster and safety brief then play to 12 or 1, break for lunch and then play for 3 more hours. So basically like 5-6 hours tops but you can always take a break in the middle of play for water and stay in the staging area for like 10-15 mins or whatever.
Neck protection is a great idea, in fact I run a shemagh to protect my teeth (loose fitting material will soak up the BB's energy and should protect against tooth chips) and neck protection. Gloves are also a good idea to protect against knuckle shots and pinky shots (like when you dribble a basketball weird and your pinky just gets destroyed).