It depends on how serious you want to train. First, I would recommend sticking with just single mag pouches (three or four). They are slimmer and you avoid the bulk, extra weight, and cumbersome feel if you start doing more dynamic drills with double mag pouches.
Reloading takes a lot of practice. Are you planning on just doing tactical reloads (keeping them in a mag dump pouch or replacing them)? Combat loads just let your mags fall...it's important depending on how you want your kit.
I prefer open top, solid retention magazine pouches. There are some good quality ones (ESSTAC, HSGI, etc.), but you will pay more. They are robust enough for going prone/falls, but retain magazines exceptionally well and if you do a tactical reload, they're easier to replace/reindex (if that's how you'll train). The basic nylon pouches are extremely hard to re-index a magazine (which is why many just dump the mag). I simply prefer the open-top retention pouches as it's easier to train with for me.
As to the platform, you can buy the all-in-one kits, or get something like the Tactical Tailor MAV (there are other types) which has MOLLE/PALS webbing, allowing you to customize your set up.
I'm moving more toward some of the micro-rigs (2-4 magazines), just to keep them more svelte and lighter (Haley, Spiritus, etc.).
Don't overload it. It's easy to do and some kits are designed for mega combat loads. You can do a lot of training with just a few mags. The chest area is easily accessible, but the larger they get the more problematic when going prone, certain shooting positions, moving, taking cover, etc.
ROCK6
