Couple of comments:
Powerex 2700 are a great choice. However, if the external battery pack is not used often then you may want to go with LSD type NiMH AAs like Eneloops. Depends on your usage pattern.
I would suggest a decent AA charger like BC900 or BC700
Try not to drain the external pack with AAs completely. AAs will last longer if they are recharged before they are completely empty. Using 2 sets of 4AAs and simply swapping them betwee the pack and charger would work well. That way you never wait.