The big 3 should be your first priority. Next you should get in on the Holiday sale XS Power is having and get a new front battery and probably a decent size one, maybe a 3400, for the rear. For 2500w this should sustain your voltage fairly well. You also need to use 0 gauge wire, 4 gauge will do but you will end up going back later and doing it with 0, so you might as well do it right the first time.
When it comes time to build your box, I would do something around 7-7.5ft3 after displacements since you are only running 2500w to it. You want to go with the bigger box because you are going to be seeing 2k at best after rise out of the Kicker 2500. I would also use 16-18 in2 of port per ft3 and tune 32-35hz.