Err wouldn't the Legion provide all that, I don't think that Caesar is going to charge his own soldiers for food, ammo and healing stuffs. They'll probably have bands of slaves go out in droves to pick the xander and broc, the slaves also prepare it for the Legion (I'd say that this is [censored] stupid but this is the Legion right?).

Can't provide what you ain't got right?
What do you do then? You buy it, from merchants, like Dale Barton.
Who would have thought Legion and money was hard to understand
Do they just pay their merchants with back rubs?

I'd bet they'd still take stuff from fallen enemies whether they needed it or not...look at Centurion armor for reference...hell even the NCR heavy troopers do it