There are actually four factions to choose depending on "style"; The Achaemenid military machine was highly diverse and included combined, and sometimes compound elements. The Seleucids bear the historical crown of succession and they do retain the diverse formula quite well; The Arche Seleukeia features, no doubt, the most vast roster of units possible to field, fostering heavy cavalry, chariotry, elephantry and the newest addition of phalangite warfare. As such the spirit of diversity and the idea of tactical "completeness" and option is the most secure in this faction.

Hayasdan, thanks to its reforms will inherit almost everything that Pahlava may muster, including all Iranian infantry, and almost all Iranian cavalry, and will field a vast spectrum of infantry and cavalry to fill the military need. This idea is however highly hypothetical (Though not unlikely) and plays more on Armenia's Orontid/Yervanduni/Arvandîg dynasty and their Achaemenid affiliation, the expansionism of the Artaxiads, and Armenia's rather oft-repeated status as a "Persianate". Like the Seleucids they will play on the diversity factor, though less, and more emphasis on Eastern warfare, but also bringing up their own flavours of warfare, in particular their unique heavy infantry.

Pontos is another candidate; They may be considered the midget of the Seleucids with an identity crisis . They also have got a wide range of troops, some emphasis on heavier cavalry, heavy infantry in the form of swordsmen, hoplites and phalangites; What is more hallmarking to them is their interaction with the Galatians, giving another flavour to the troop diversity not previously present in the Achaemenid worldly order.

Last, but not least, the historical torch-bearer of the Persian Empire; Pahlava. While they are entirely all about horse-archery and heavy cavalry, they were not strangers to infantry at all, but restricted them for mainly garrison duty. They will get everything the Iranian Plateau will offer by native terms, so they too has the basic availability of local diversity, like the other factions; Less so in Caucasia, and entirely scrapping chariotry, fostering traditions of archery and equestrian arts. What they lack in availability of Celtic troops and foreign unique shock elements, they get in additional supplies of nomadic horse-archery, heavy cavalry and an extended availability of Indian troops. It is far easier to chart this out through cultural spheres in a map. In my opinion it is a matter of taste; The AS and Pontos provide a Graeco-Macedonian and Galatian twist to the Achaemenid military machine, Hayasdan provide a plan of preservation in accordance to their own local customs while the Pahlava almost entirely focus on two equestrian paragons for field battles, retaining Achaemenid-style infantry for garrison duty.

There are of course factions like the reformed Saka or even Baktria, but I think of them as more unique and rather more "influenced" by the previous system rather than inheriting it.