As a CSA, I use to purchase big brigades (for example the 10 units in VA at an overall strength of 168).
OK, it will count as 7 units when forming your div but each unit is a little army by itself (ie Infantry, Cavalry and Artillery), and it is cheaper than purchasing each element alone.
With 2 of these brigades + 1 leader (always put a leader on Division especially with special abilities) + 1 HQ Div + 2 artilleries (purchased individually), you have a powerfull division in one month and the 18 boxes are filled.
Doing so, you will be sure to have a well balanced division with every arms on it, and the 10 brigades you can purchase allow you to have 5 divs like that...
After that, I would prefer to have numerous small corps (ie 1-2 divs) rather than 2 big corpses in a given army. Doing so, you will be able to have a military presence in a lot of adjacent areas (more flexible), and corpses can help each other because of their abilities to follow the battle sounds (rough translation from french, not sure of it...

).
If possible recruit locally: units better fight on their home state, and you will save both Rail points and time to move your units where needed. Not possible for HQs, I know

leure:
Of course, this is general comments, and sometimes I don't act like that because of particular situations (ie more divisions when besieging a huge town in order to breach walls for example).
Another thing to keep in mind is to have an entire army/corps/divisions structure: This gives bonuses (or at least you don't have malus. It is especially true for the CSA who terribly lack of HQs)...
And the more important: Experience by yourself
Hope this helps