The Parthians aren't quite Persians, even though they settled in the rough area as the Persians did. I think it was Steppe Merc, but I can't be sure, who said that they were nomadic, and so explaining the different style of fighting (horse archers and heavy cavalry, as opposed to a whole bunch of ragtag peasants with assorted 'professional' troops from throughout the whole empire). The Sassanids are Persians, but I don't think enough people have BI in the community who have time for a PBM.

Some quick browsing through the TW communities I linked you to in the last post should give a mod for RTW that fits your time. Or, better still, find you a time that you think is more interesting and that you think others will find more interesting too.