The rebuild kit I just got from Fastlane was enough to do one caliper. If you want to do the left and right units, you'll need to get two...but the one kit will do the whole caliper, not just half of it.
You might want to get the pistons out before getting the rebuild kit, though; if the pistons or bores are too rusty it would be better to get another caliper. In fact, I should have replaced the caliper, and I probably will do both next time I do the brakes...
You say the pad was hardly worn at all? If that's the case, it would make sense that you couldn't really push the piston in; there is only a tiny bit of travel left when the pad's almost full...
Just some thoughts...good luck!