I've just checked the prices of 5-6S LiPo and they seem expensive for a first build. I reckon it would still move on a 3S, it has been running pretty well on an 12V drill battery in testing.

Considering that I only bought the 20V motors because they were cheap in Aldi and Argos only had one drill in stock at the time, I may ditch them for some Argos motors and run on a 3S. I already have one Argos 12V motor in reserve, so would just need one more. They would be easier to replace in the inevitable event that they break and it would be less hassle to get mounts made too.