yeah 2 cell lipos is the way forward. Just make sure your ESC has a lipo protection setting on it, if not you will need an external one.
You will need lipo battery, lipo capable charger and lipo protector if it not built in.
when you upgrade to brushless its most likely the brushless esc will have built in lipo mode.
when you switch to lipo batteries you will notice more punch.
when you then switch to brushless motor with the lipos it will be like a completely new car. much faster and punchier.
I started with a second hand brushed monster truck that was well ******* not great.
put a brushless and lipos on it and it jumped from 20mph to 50 mph and turned into a wheelie monster, trouble was the car I had just wasnt strong enough and kept breaking.
At least with the firestorm you have one that is designed to be brushless and has a good reputation for withstanding the high speed crashes you will have pushing the limits.