I am very happy with the MMM. I am using a Neu 1512/3D motor with it on 5s lipo. The main difference between the Tekin system and Mamba Monster is that the Tekin system is sensored on start ups, so it starts much smoother than a sensorless system, such as the MMM. Ideally, I think I'd prefer the Tekin RX8 system, but because I do not race very much, the Mamba Monster works great for me.
As for batteries, 4s, 5s, or 6s, will work great in a buggy as long as you choose the right KV motor. 6s and 5s is definitely more efficient than 4s, but is also more expensive. On 6s, look for a motor with KV between 1300 and 1700. On 5s, choose a KV between 1700 and 2000. On 4s, choose a KV between 1900 and 2500. Of course this is all relative, but ideally try to stay between 30,000 and 40,000 RPM. And a 25C or higher battery. I am using a NeuEnergy 25C 5s 4300mah lipo pack, and so far really like it.
This calculator by BrianG helps a lot in picking a motor, voltage, and gearing.
http://www.scriptasylum.com/rc_speed/_top_speed.html