Tim: Since you seem to be uncertain about whether a tabletPC would meet your needs, or not, I'd like to suggest that you try one before you get too serious about finding a permanent long-term solution.
As has been mentioned many times in the tablet blogosphere, Allegiance Technology Partners ( www.alltp.com )features a 48-Hour demo program that allows you to "rent" a tabletPC for 48 hours. All you pay for is the shipping. However, in your case, I'd suggest that you consider buying a used tablet so that you can try it out over a somewhat extended period. With luck, you might be able (probably will be able) to get one with OneNote as part of the package. It definitely sounds like you need a good mobility solution, and having been in ranges myself, I know that environment can be demanding.
As for what to look for, why not try the Lenovo ThinkPad? It's a popular, and it will give you a good idea of what current model tablets are capable of.
Tablets really are a paradigm-shifting concept; I strongly advise you to take some time to explore this before you try to jump into the pool (with the rest of us)...