Flight box battery

You don't say how big the space is for the battery, in the box.
For most people, a motorbike battery will suffice 12v and 7 amps will last all day, generally.
You will also want a couple of screwdrivers of different head styles and widths, a glow-plug spanner, spare plugs, and a small spanner - an adjustable one is fine - for undoing prop nuts, and spare props, because you are bound to break a few!
As you are asking a pretty basic question, I assume you haven't flown r/c models before, so I strongly recommend that you join a club near you, and learn properly. It will save a lot of heartache and broken models!
You will also need insurance, and a CAA identity number. The club will probably sort that out for you.
Then you will find a spray bottle of degreaser, kitchen tissues, and the like, for cleaning your models after flying - will be useful..

The DR7 you mention only has very small 'spade' connectors, so would be very fiddly to use on the field.
 

Most commercial flight boxes are sized to take the 12volt 7Ah alarm system battery (it's a leak proof Gel cel) They're very sensitive to over voltage and over current charging, a standard car charger will destroy it in fairly short order! Also, always recharge it immediately you return from the field, leaving it in a discharged state will also seriously shorten its life.
As an aside, since the advent of the new small capacity UPS which use this same battery (1 or 2) it's become impossible to find any batteries of decent quality. (Locally anyway)
A Lipo equivalent is available at an eye watering price but not to be recommended unless you have a proper charger.

There's no such thing as a daft question, only stupid answers!

Er?