I wouldn't run just one battery pack on a jet just for the backup reason alone. Typically a 2cell lipo will run some where around 8.5v or so right off the charger so you will need a regulator that will bring it down to 5.6 or so. (unless you are using servos rated for the higher voltage) Also, you should worry about the peaks of a typical flight. If I remember correct a typical flight of scale jet like that might peak several times in the 10-15amp range depending on style of flying but the average consumption is around 2-3amps through out a flight. I would at least run 2 2400-3000mah or so 5cell packs if using nimh or nicad. If you choose to go Lipo I would run 2 2cell 2400-3000 mah pack with both packs on regulators.

You will have a good bit of time, labor, and money tied into this jet. Last thing I would cut back on is batteries/servos/switches. Optimally you would want to run a power distribution setup like POWERBOX etc etc. The are other brands available but most of them have built-in regulators, dual battery support, multiple servo support per single channel, and complete redundancy for a minimal weight gain.

You will have quite a bit of servos in this jet. 2 ele, 2 flaps, 2 ail, 2 rud, 1 ster, 1 brk, 1 retr, 1 dor cyl. That is a minimal. BE SAFE NOT CHEAP