As far as the pistol choice goes - i don't know much about it to offer an opinion. Your listed retailer does have the King Arms M4A1 'full metal', yes.
I'd suggest removing the retailer link in your post - generally linking to retailers for guns outside of the restricted guns discussion sub-forum is frowned upon - it's an AV thing.
Airsoft weapons generally are scaled 1:1 with the real deal (ie if they are a counterpart to the real deal) so magazine pouches, holsters generally will work the same.. some variations in weaponry features like under-barrel 20mm rails may cause fitment issues on occasion, however.
If you have a rifle for a primary weapon, and are using the pistol as a back-up, then treat it as such - 2-3 magazines should be sufficient for carrying on person. More magazines as necessary, if you are relying on it in more CQBish situations. I get by with 2 magazines in the outdoor field environment that i'm used to, so far.
When they are referring to hydration - pick your own poison. They are referring more to the method of carrying liquids - the more common method is usually by a 2.5 litre bladder carried on person in a 'camel-back' or backpack attached to your webbing or chest-rig. Hydration when playing is one of the more important aspects of playing - if you are running dry, it's like running without ammo.. you are pretty much useless to yourself and others, dehydrated.