Check out Charas Project for tilesets (or is rm2k called chipsets? forgot). Got lots of crap there.
Well, as stated by various people in various forms, the thing you're trying to do has been done. Therefore, maybe you should start thinking out how you're gonna make it immersive and original (or at least the theme with a twist, which today is the same thing as originality). Is it assumed to be the current earth's direct future and not some kind of alternate where you just set up the holy roman empire easily? If it is, then plot out a history of the world a bit, for general references to the player about this past. Assuming you've got the general secret the christians trying to cover up thing idea done.