Re: How do you determine what technologies to research first?
Yes, Rome II tech trees include techs to recruit agents.
I agree that it doesn't seem historical that you'd need to research recruiting a champion. Everything in a strategy game doesn't have to be about representing historical developments. This seems reasonable as part of Rome II's system of culture-specific tech trees. For example, the Iceni get champions with their first military tech, whch sounds reasonable, considering that Celtic cultures venerated champion warriors. Rome takes a bit longer as they need one military tech beyond the first one to recruit champions.
Adding a champion improves units slowly over time by giving them experience, when the agent remains in an army for a a decade or so. It seems reasonable that, if a champion warrior trained soldiers for that length of time, they'd be more effective. I like the way that the quality of an army's training (as well as its equipment, traditions and commander) all make a difference in Rome II.