I would suggest a Mamba for it. Either the 6800 or the 8000. I am running the 6800 on 6 cells and it is so much fun. It is very fast, yet still completely controlable. As for lipo, Max Amps sells a 3s lipo battery designed specifically for the rc18 series.
http://www.maxamps.com/index.html?lmd=38519.529109