Mechanical brakes definately aren't required. They are included in the Ve8 kit, but obviously, don't have to be fitted. The main advantage of mechanical brakes is that you can set front to rear bias.
However, they are heavier, and the extra servo will draw a little extra juice, which balances out the ESC braking generates heat argument. I definatley don't think that mechanical brakes give a run time advantage over ESC brakes considering the extra 80g of weight and drawing extra power.
I would run the 2650kV Mamba on nothing more than 3S to be honest. On 4S, it will theoretically rev to 39220 rpm, which is a little high if you're after maximum efficiency and cool running. Not only that, gearing options will be more limited at a higher rpm. If you are looking for maximum run time, and aren't going for insanely high speeds (just a little more than a nitro), then a 2200kV motor on 4S is better suited.
Gearing will depend on the voltage used, check
my beginner's guide for details.
The Mamba Monster has been around for a comparatively long time now, so I'd maintain they are the most reliable option out there. The Castle-Neu motors appear the best value for money right now in the absence of Medusa.