Remember, you can not only vary the voltage of the battery, but also the capacity.
A 4S 5000mAh has the same power (read same run time ignoring efficiency) as a 5S 4000mAh pack, and as a 6S 3333mAh pack. All will be about the same weight and price. The 6S (with an appropriate motor and gearing) will give the most run time - it is the most efficient as the total power is the same as the 4S set-up, as the current has reduced, so the heat build up will be less - less energy from the battery is wasted as heat.
Any of the motors will be more than fast enough to run with the nitros to be honest. The 1900 will be a little calmer on 4S than the 2050KV. On 5S I'd pick the 1550 if it is a smaller track, and the 1700 if it is larger. On 6S, I'd go with the 1350.
If you have a read of the suggested rules for electrics in the UK, we are to run 10 minute finals (and have two legs - results added together).
The RC-Monster mount isn't what makes the mech brakes a problem - it has mounting holes for the pads, it's the chassis - it was never designed or drilled for the second servo required.
Voltages wise, I have run 4S, 5S and now run 6S. My current packs are 2500mAh 22.2V (6S). These are good for about 12-15 minutes racing on the track, with my motor, settings, track and driving style.
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