I agree you need the big one, but are you sure a messenger bag is the answer? Sounds like you'll be carrying a fair-sized load pretty consistently, and at least for me, a messenger bag dangling from my shoulder is a PITA on any ride longer than half an hour, max. Even the good ones (I have a Patagonia) don't ride as securely as I'd like, the shape isn't ideal for clothing or bulky things, and they're sweaty in hot weather.
Most of the time I use a Carradice Longflap seat bag, sometimes with a Rivendell Baggins Hobo Bag on the handlebars. Here's a link to Carradice:
http://www.wallbike.com/carradice/ca...addlebags.html,
Riv doesn't have the Hobo Bags anymore, but this is similar:
http://www.rivbike.com/webalog/baggage_racks/20128.html