Personally, I prefer to buy only full packs for that very reason, and I never seem to have regretted having extras for this or that project close at hand. However, for people who might be doubtful about the scale and only want to “build what they buy,” to contain a project like this, it is an option.
My Bordurian and Syldavian armies are a proof of concept for this, since it’s experimental at this stage. My Bordurians have an armored regiment with an attached motor rifle battalion (abbreviated and with less equipment, as befits a second- or third-tier power), a recon unit, and an armored artillery battalion. Total clock-in is $21.40, with no extras.