what to build first

[athanaric]

With initial money, I always spend on economy if I can. Roads and Farms are highest priority - fast troop movement and more growth for your towns, handy at the start when you are forced to put taxes on Very High.

Roads, farms, ditches. For example, SPQR should build paved roads in Latium with their starting money. Most barbarian factions should build a ditch and a farm upgrade in their starting province, unless those already exist.
Don't forget to build healers later on since they're very cost-efficient (+1% health for 1600 mnai) and give your FMs good ancillaries and prevent plagues.

For only two factions, I hire mercenary troops with starting money. Koinon Hellon: Hire unit of Cretan Archers - worth their weight in gold! For Saka: Hire a unit of cheapish Sauro horse-archers to help take town in south-eastern desert quickly, since it takes ages to send reinforcements through the mountainous terrain.

For Pontos, hire a unit of mercenary Botroas (Enoci Curoas) with your starting money. You're gonna have to blitz some provinces anyway and all infantry units of your starting army are horrible at assaulting walls or flanking. Botroas fill that niche perfectly. Plus they have better lethality than any of your early troops (except cavalry bodyguard spears, but those aren't really appropriate for drawn out melee).