Hmm, as long as you cover the bases I think you are quite okay. My choice would be something like:
- Cleric (or oracle or warpriest), preferably of Pharasma: there will be plenty of work for them, and Ustalav is Pharasma country
- Bard (or inquisitor or evangelist cleric, maybe even rogue): there are times a bit of flair and taste work well, and they can support the group
- Barbarian (or ranger, paladin or inquisitor): there will also be times when you need to bring the smackdown. Knowing your way around the place doesn't hurt either.
- Wizard (pref. necromancer, diviner or transmuter, alternatively sorcerer or witch): Knowledge and arcane power will both come in very handy now and then.
- Most any fifth character works well when there is some synergy in the group. I would go for an alchemist for the versatility and some mad scientist vibe (why let the NPCs have all the fun) or warpriest for the crusading against the darkness theme, but really, knock yourself out here.
As for races, go with whatever works with your concept, but keep in mind Ustalav is fairly xenophobic and non-human races are not typical (and often disliked). A possible party would be:
- Alchemist (elf or tiefling, possibly vivisectionist)
- Warpriest of Pharasma (human)
- Barbarian (human, HElf, HOrc, or dwarf)
- Wizard (gnome or human)
- Bard (I'd go aasimar or human dirgesinger bard for style, but the base is pretty good too).