[PlasmaKrab]

IMHO how it turns out in the long run would be as much a matter of international politics, e.g. which coultries support which side. You could actually have either hard-pressed rebels or a staggering central government supplied from overseas, though I don't know if they would need that much.
F.e. a total embargo on oil products would end manoeuver warfare and air combats quickly enough, except if some domestic oil industries are left standing.

As far as captured units are concerned, I'm not sure there would be enough left of them to build a fighting force. AFAIK there should be a couple dozens each of several Russian tank types around, same for planes, helos and arty, most without much ammo, and more importantly, without many trained mechanics. Stuffing a new gun in an old tank is nice, but that's not something you do in the field with a blowtorch and a disk saw, paticularly when you don't know the vehicle itself.

Besides, refitting captured Iraqis early-80s T-72s to fight Abramses would be suicidal.
Deal with that as you want, maybe I'm being to rigidly realistic here, but I'd tend to see militia forces striving to capture active or reserve (regular or NG) equipment and bases, as you said, and/or sticking to local vigilante-level irreg forces, most of which armor would be made up of civilian trucks with makeshift armor and pintle-mounted weapons.

Anyway, if you want to go for the captured-refitted thing (or units supplied from overseas countries and refitted, sau surplus Challengers, etc.), you just have to buy these units as captured and then give them the weapons and ratings you like, all in the scenario editor.