Max for me if you are sticking to the concept of a small handheld the hassle of getting a power cable and battery arrangement is just too much. I'd do an Ebay sell/buy and get one of the Garmins similar to mine that don't eat batteries. My Etrex Venture CX uses 2 Duracell AAs and they last ages, on a long day out I carry 2 spare batteries but often they aren't needed.
Units like the 276 which I've considered really need a backup 12v supply for when their own battery goes flat, and an older/used one might not make anywhere near the 10hrs.
Another advantage of an upgrade in the small portable would be to get one with mapping. I really wouldn't want to be without it (and yes I do have laminated chart sections and a compass as a backup).
My unit is like this... http://www.adventuresofgreg.com/HPB/...653-738212.JPG