Your NiMh pack will work well. It's obviously limited by the capacity and quality of the cells but, for the length of time you're intending to operate, they'll be absolutely fine. The radio will default to a maximum of 2.5 Watts when the battery voltage drops but you can force it back to 5 Watts by cycling by through the power settings until the power output icon blinks.?
I have used the "LADDA" branded NiMh cells from IKEA. They're identical in performance to Eneloop Pro cells but cost considerably less. They're made in the same factory in Japan as the Eneloops.
