Well, the general method is to see what army the real life nation your NS africa nation's territory occupy and then build it from there. Fx. i started out with a small army with obsolete equipment like a fitting a small african state, but I've gradualy built it up as a RP'ed my nations rise status and wealth. Slowly replacing older mnilitary equipment with more modern. And of course by looking at the general wconomuy of your nation you can sort of figure out what would be realistic.
Hope this helps you a bit
However the War Mod should be better able to answer your question.
