Welcome to the forums! You sound like you got it all together. You should definitely try to squeeze your cells in there first, no reason to drop more money just yet. But if you do decide to go lipo I suggest you take a look at TrueRC's (great bang for the buck) 8000 packs for runtime and their 6400 packs for a lighter setup. They're about half the price of other lipo's and so far I have yet to hear anything bad about them, so long as you don't draw too many amps(their C rating is lower than other companies). With them being so affordable you could buy 1 4S pack and 1 5S pack and give them both a try.
Also, you should look at BrianG's calculator if you haven't already, here's a
link, it's really handy to have. I think a 12T pinion might be a little small for a 9XL, even with a MX style track. You might want to invest in a 13, 14, or 15T so you have some options.
Good luck and keep us updated!
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