Word of advice:
During my tour, and most other tours, the use of after market protective equipment is not allowed. There are many articles on these forums regarding reasons and examples.
From my personal experience; A fellow soldier in my section had a 'bullet proof' vest his parents gave him as a gift, our platoon WO told him that he was better off leaving it at home, since he still had to wear issued gear.
Essentially, you can buy what ever protective equipment you want, but you still have to wear the issued helmet and vest. Thus, buying a second vest would be pointless as the issued one still has to be used. I doubt any soldier would want to wear 2 vest during operations.
As for goggles, gloves, and tactical equipment; the use is dependant on the CoC (Chain of Command) and the standard warnings apply; If damaged, lost, stollen or destroyed, the military will assume no responsibility. If it becomes contaminated with blood, it will be destroyed (burned). So it becomes a 'use at own risk' situation.
I wouldn't advise buying any body armour unless the soldiers CoC has specifically cleared its use... but I doubt that will ever happen.