Sounds like you're doing your homework, and are doing a good job of planning. My first tour was much like one you are planning. If you haven't done so yet, you may want to take a look at the 'How To' section on this site.
For my first tour, I bought a set of cheap, small panniers. I have since bought a 'good' set for expedition touring, but my original panniers worked well, and I am still using them today for running errands and sometimes, for commuting when I need to carry extra stuff to/from work.
Let me suggest that a set of panniers on a rear rack will work well for you. See http://www.rei.com/product/749997
as an example.
Panniers will be easy to pack & unpack, will keep the weight low, and will allow you plenty of flexibility in your travels. It's surprising how stuff adds up when you think about carrying a change of clothes, first aid stuff, toiletries, tools and so-on.