Is a computer battle game out there where instead of creating knights, archers, pikemen etc. you have a certain amount of supplies and manpower, and you can arm you soldier however you choose?

Eh. Spread across several differnt games. You can kind of edit your own units in SMAC.

Supplies and manpower are modelled in Hearts of Iron (a WW2 simulation). However, the units are predesigned (quite logical since we're talking about a historical game.) :p

Superpower 2 might be somewhat like that when it comes out.

Well, Hidden & Dangerous lets you choose from a varied group of soldiers and arm them how you wish.

Moved to Other Games.

Hm, you could try one of the Settler series. Though I believe there aren't different type of soldiers, but you are limited to the supplies available.

In Perimeter, a new game, you can change your soldiers to different types on command. Costs plenty of money though.