Romani are by far the easiest faction; Kart-Hadash is well set, but with so many provinces can be puzzling; Lusotannan could be a nice choice as they are isolated, but you'll be facing serious enemies and tough fights...

I say start with the SPQR, just to get acquainted with the game, then choose whether continue or switch to the Luso...

In the end all you need to get used to are the battle balance and the provinces management, after you can move to role playing to get your experience more entertaining ^^