ZimRathbone is absolutely correct that as always in Aurora, there is no correct answer because "it depends..."
NPRs do not build PDCs so you don't have to worry about them. But an NPR might have an absolutely colossal ground army waiting on their homeworld and unless you significantly out-tech them, you'll need several divisions of HVA plus dozens of Garrison brigades to pacify the population.
I usually have 4 Ground Force Training Facilities that are always active, so I build 4 divisional headquarters, then 16 brigade headquarters, then enough heavy assault/assault infantry/mobile infantry battalions to fill them out. Generally I build two types of brigades, heavy brigades with 3x heavy assault and 1x combat engineer battalions, as well as light brigades with 2x assault infantry and 2x mobile infantry battalions. Because I do conventional starts, I have enough time to build up the four initial divisions before I need them.
Don't overlook the need for Garrison battalions to pacify conquered populations as well as the cheap REP battalions to bring your expensive combat units back to full strength.
As for transport, I would recommend two ships: as fast as possible single battalion transport for those cases when you urgently need to capture a listening post or something, and a big divisional transport that can move 22-25 battalions at one go: a DHQ, four brigades and few REP.