Well thanks to most of you I've just bought Medieval 2 Gold Edition from OzGameShop for $13. It would've been about $3 cheaper on Steam but I avoid Steam when I can. The Medieval period is a lot more interesting to me than the other non-Rome ones.
Do the Rome mods add a new campaign, or do they replace the existing one? So if I want to install one of them (Europa Barbarorum, Roma Surrectum), should I only do it once I'm done with the vanilla version?