You only need three units, really. Drapanai, Komatai, and Scythian Foot Archers (regionals, readily available in Kallatis; if you're cheap, you can go with the standard Komatai Toxotai) in a roughly 1:1:1 ratio, plus a general. For the price, this combination is stupendously effective both on the offense and when defending, and it will easily trample anything fielded by the AI (except maybe a full-stack of horse archers, but why would you travel to the steppes, anyway?).

For order of conquest: in all three of my Getai campaigns, I started off by capturing Sarmizegethusa, then Naissos, then Kallatis, and then Tylis. At that point, you have an excellent base from which to roll through Makedonia (your Drapanai will absolutely shred their pikemen) and then Hellas.

Economically speaking ... meh, there's not much to do but bide your time until you're able to capture the juicier settlements to the south. Once you manage to climb out of the red (you'll almost certainly fall into debt at the beginning), set taxes in your home settlements as low as feasible to foster population growth and build a nice tax base.