You want to get atleast a 40W soldering station
. Not a soldering gun.
A good product such as this is a Weller WLC100
. <-- That's also the cheapest price around.
PS: Just a question that I personally do not know myself: Is 40W enough to assemble batteries? I mean you wanna be quick about it not to ruin the cells, so should one bump the wattage up to 80?