First I think you should probly choose the kind of BDU you want. Then the style of vest/chestrig and match it to your bdu.
As for mags, probably everyone will tell you not to use any hicaps. They don't work very well, sounds like maracas, and it's kind of unrealistic to have 300 rounds in a single mag. Personally I don't want to exceed 90rnds in mine. I had (lost some mags somewhere O_o) 4x90rnds and 2x30rnds. It is more than enough for me. Depends on how much you love to shoot (read "waste BBs"
but that's ok too)
I'd highly recommend a good 4-5W UHF radio (check with techseller on Asc) and headset+PTT
For all the rest it reallu is ypur preference