Should only need a plug soldered on it to match your batteries, some servo tape to mount the ESC, and a 48 pitch pinion gear or two. Check castle's site for gearing recommendations, and be sure to check the motor temperature frequently after installing it. 150-160 after a good run is about the sweet spot. I don't run mine past that. If it's getting hotter than that, you'll need to go smaller on the pinion or larger on the spur.
I screwed a flat piece of Lexan left over from trimming a body out on to where the old XL5 was mounted on my son's. Made a good mounting point to tape the ESC.
Remember the castle stuff (and most other aftermarket manufacturer's stuff) is NOT waterproof, but then again neither are the bearings. Keep it dry and the temps under 160 and it should last you a long time.