Hey Pedro,
I'm a huge Tamiya fan, but this site is not really geared towards the Tamiya crowd. There is a good Tamiya forum at UltimateRC.com.

I'm curious as to what "badass" means to you? Performance? The Grasshopper isn't going to offer much in that department no matter how much you spend. I'd just add a bearing set and build it.

The Grasshopper kit should have come with an ESC, unless you bought it from Japan. The ESC is a decent one so use it. To add a 540 motor, you'll need an 18T pinion (32 pitch). The Grasshopper chassis can't really handle much more than a Tamiya Sport Tuned so I'd get one of those. Any cheap servo should work fine. Don't waste money on a fancy one. Unless you really want to run a lipo pack, this car would run just fine on a 6 cell nimh pack. They're really cheap. A 3000 mah pack should give you 20-30 mins of run time easily.

Here's a picture of my Hornet. Same chassis. It is one of my favorite runners. It's got a Sport Tuned motor and it's a lot of fun to drive.