The Mega 550 is a brushed system factory so you would need an esc as well. A good, cheapish solution is to buy the factory spektrum brushless stuff from jennysrc.com. Outside of that, the options are pretty much endless.
There are a few other parts I would upgrade as well that likely won't tolerate the extra power of the brushless system. The spur assembly and the center drive shaft. For the driveshaft, you can just add the center support and bearing that comes on the factory brushless system. For the spur, you would need the whole assembly. The truck will work without the two parts I mentioned, but not very long.