Thanks for the advice about the adventure in general. I certainly don't want a TPK (my beguiler needs protection when he gets back after all). It does seem like the main problem is in balancing having some kind of plan as to how to make this fun/survivable with the openness to allow the players to improvise how they see fit.
Gnolls do make good bandits, but I think they'd eat a little community of halflings rather than demand their harvest at a later date and give them time to recruit a bit of help (also, we're fighting some in our current adventure that are helping an evil baron).
I think I'm looking at using a group of Shifters, as I do like that animalistic vibe, or Tieflings, which would be fun for the Paladin. Both are 1/2 CR for Warrior 1, so the squads the group would fight at the correct EL would be 10-12 under ideal conditions for a 4th level party of 4 (if things were to proceed along movie lines, I'd assume they'd fight off 3 squads the first day, one broken up overnight, and half again as many in the final battle for a total of over 60 bandits) . Part of the idea/problem is that defending the village with just seven samurai four adventurers is supposed to stretch things a little thin, those squads wouldn't go to the same place over and over and (again, if things progressed like the movie) would test out all of the fortifications (possibly splitting the party in the process- though it's likely they'll have a group of villagers with spears to help out).
The leader is going to be a sorcerer/rogue with a high umd score, who I think will have a wand of something that they had stolen from somewhere (enlarged magic missile?) that will replace the threat of the rifles from the movie. (Basically that the enemies have a way to snipe at you from far away if care/cover is not taken). Does this sound reasonable or is the threat of defending a village enough without adding more?
Are there any rules anywhere about building basic fortifications and how long that takes? I want to give them time to prep the village, but they shouldn't get to turn it into a keep. Just what they and the villagers could do in the time they have to prepare (something like x wall units a day, y ditch units, ect)