[@Sven the Silent] I would assume that a lot of the combat problems would fall onto the players themselves, generally because we wouldn't want people doing something that's absolutely inconceivable with their units (Like flawlessly blocking a volley of arrows)

I'd assume that the tribes don't really have much of an 'economy', but use more of a barter system. (Of course I've never done a Nation RP so... I'm not really helpful am I?)

EDIT: On the note of units, you could use a rock-paper-scissors kind of system.