If the yeast that comes with it is the Cooper's dry yeast, I'd just go ahead and use that. It is a good yeast imo and I use it in one of my brews. If you are concerned about the vitality of the yeast, first off if you bought the kit from a reliable place I wouldn't worry about it. If you think it may be old, any of the following will be fine:
Nottingham
Safale US-05
Safale S-04
Cooper's
One pack should do fine. Although, with the Cooper's you might want to pitch 2 packs as they are 7 gram packets as opposed to the 10 to 11 gram sized ones (this is the standard size). At any rate, I'd suggest getting an extra 2 packs to have on hand in case. They will keep a very long time under refridgeration.