Your best (and easiest) bet would be to simply roll up some standard characters. Create a range of 'plebs' to work as the majority of the 'army'; make them versatile. Create a template for mid level spell casters and archers, to fight from a distance. And then can create a standard 'assault group leader' and high level spellcaster/rogue/bard/whatever suits, that appears within each group they fight. And maybe one of two high level 'rulers' for the cult that the group has to battle against to win the day.
If you wanted to include monsters, let me know what sort of cult. Actually, I could likely roll up some characters for you too.
I've created a database system that lets me instantly roll out full-fleshed characters from the base PHB (the skills are a little spread out, but it's too hard countering that). You choose a level, race and class, and out comes a character with a name, rolled states (with increases for abilities due to level; class specific), feats, spell lists (including memorised and known spells), gold, normal items, magic items (according to what would be expected for someone of that class/level), etc.
As for monsters, I've got a database with a good number of monsters inserted (more going in regularly) that allows me to specify qualities I'm after; such as CR, HD, environment, special qualities and attacks desired, etc, and it spits out suitable enemies; ranked according to how readily they meet my criteria.
If you wanted, I could spit out some details and post them in here for you. (I'm off to Uni soon and I've got a game tonight, but I will be home for a short while between the two, otherwise I can do it tomorrow)