[Zfurlong]

helmets, I don't use them, I run a baseball cap.
batteries, 2-3 batteries should be fine (I have like 6 myself varying in size from 400 mah to 6500 mah (that last one is primarily for bigger guns)
as for a low voltage indicator, I don't have one. I just use my voltage reader. if a battery starts to die, I swap it out and put the dead one on charge.
when it comes to misc gear, a chrono is the one thing that I wish I had before I started doing my own upgrades. otherwise you will need to wait until you get to a field to test your guns. other than that, you should have a variety of barrels, hop rubbers, nubs, and smaller parts on hand to make quick repairs at the field. pistons, spare gear sets, piston heads, cylinder heads, tappet plates, springs, cylinders, wires, heat shrink (shrink tubing), varying weights in bbs (especially for support weapons, I mostly use a 1:2:1 mixture of .28, .3, .32 bbs pending the type of field I am playing at on a given day. if there is little to no brush and a lot of open field with man made barricades, I will use a 1:1:1 mixture for a good spread, if I am playing on a field with a lot of thick, heavy brush, then ill use anywhere from a 1:2:2 to 1:3:3 mixture of bbs). when it comes to the mixture of bbs for support weapons, make sure that you are using the same brand and same kind of bbs for the gun so as to not potentially cross contaminate the whole batch (using g&g .28 bios, with madbull .3 styrene, and elite force .32 bios for example) as the different bb brands have different ratios of chemicals to make the bbs to different standards, in addition to some brands having a type of coating on the bbs for whatever reason (heres looking at you g&g).
other than that, you will need spare mags. I would recommend having 2-3 high caps on hand, and multiple mid caps (the high caps are primarily my loaner mags/buddy mags on the field if I know I am playing with newer players that day.