My beloved Brittania(RTW PBeM)

[Charge]

Faction: I would suggest Rome, Carthage or Macedon (large numbers of possible expantions/or diplomatic stances;
Difficulty: well, at least H/H, though for me VH/VH isnt enough usually. I also found battles much more challenging with morale turned off. We should play with house rules, role-playing etc..
Unit size: Large (normal or lower would not be to my preference (moderator edit), huge not everyone can handle)

As for mods, surely not vanilla, hmm, it turns what mods people have or can download or not them..