You are thinking well with the MMax pro. Its a good ESC that can work 1/8 and 1/10. Its only downside is the BEC is a little weak and very strong servos will twitch with it but for smaller RCs like the firstorm you don't need a massive servo (under 200oz-in is fine). It is overkill for a small castle motor and comfortable with a 1/8 motor, especially for the weight of the storm. It also has a small footprint compared to the MMM and RX8.
There are rebadged versions of the MMPro from all the major RC manufacturers and they go for a reasonable price on ebay. I run one in my firestorm and I also ran my losi 8ight-T with it for half of last summer.
A 4s battery and 1/8 buggy motor have doubled the weight of my firestorm, though. We will see if its practical. Its a street-only truck now and I drive it as a novelty so the weird electronics setup is idea for that. I should have it finished tonight and have the first runs on it and photos for here.
As far as batteries go, just get the highest C-rating you can afford. You can never have enough C. It is not just for performance, your ESC will run cooler and last longer.
The 3800 castle motor is extremely powerful for the storm. Plan on blowing the tires off the beads constantly. Its a serious motor for that truck and its pretty small and light.