The car is highly upgradeable. There are a multitude of parts available of varying value in terms of increasing performance. Your first considerations for upgrades should be in the electrics department, rather than the kit, though some carbon fiber shock towers would be high on my list of chassis upgrades, as I've heard that the stock stamped aluminum ones bend more easily than perhaps they should. Those who want to do extreme bashing have been known to install extra bracing, though I seriously doubt I would do that myself, no matter how extreme the bashing.

On the electrics side, you should plan to replace the steering servo pretty quickly, but that goes largely for any RTR. The motor and speed control are limited to 4S lipo battery, this is not a bad thing. Buggies do not 'need' to be able to run 6S, but it is always a nice option to have. SO there's your battery choice, you want 4S lipo packs. You want them to be capable of delivering 200A, so at 5000mAh capacity, that requires them to be 40C (current handling = duration * C rating). If you go down in capacity, you need to go up in C, if you go up in capacity you can afford to reduce C rating, but really you should consider 5000mAh at 40C as your ideal battery overall, for physical size, capacity, charge speed, power delivery, it's the best of all worlds that battery. Of course more C is always better, and you will be able to feel increases of C (car will feel snappier), up to around 300A (5000mAh@60C). OVer 300A handling you will gain little.

Another option is to run 2S packs in pairs, with the same instructions concerning current handling. If you go for this dual 2S option you will want a charger that can charge 2 packs at once, BUT you will also have batteries that can be used in other cars. If you buy 4S packs, they'll be more convenient to charge, but can only be used in 1/8th vehicles. BAsically, with a combination of 2S and 3S packs (this is how I roll personally), you have the right batteries for pretty much all the models available and can just run them in pairs when needed for crazy 1/8th stuff.

Get the best charger you can afford, it is something you should only buy once, so buy well. If your budget can stretch to it, a dc and a suitable 200W 25A power supply is the way to go, though many prefer AC wall plug equipped units, they will not be capable of the same current delivery.