EMP is not likely to be able to damage lead acid batteries.

Gel cell even more bulletproof.

Your best batteries are going to be large semi-truck service batteries. Bought locally from trucking shop,y places. Don't pay any extra shipping for specialized batteries of a lower capacity. You are better off just going big than going specialized when it comes to battery storage banks. Preferably, you want all of the batteries to be roughly the same age and type, but it's not necessary. I have had pretty good luck with mismatch batteries of the same voltage and some people are even using 12v large capacitors which come from the custom car stereo industry, putting the large capacitor in parallel with the rest of the battery side of the system. Apparently the capacitor prevents power fade by handling the spike in power needed by high amperage items that need a lot of power to initially get going.