Hi Ian, Looking like a good test bed.
Batteries, some people use 2.2amp as a good start with a good high C rating 30 minimum.

I run my 2wd Argos drill bot (with electric axe) from 4s 2.65 turnigy nano-tech 35-70C and it lasts a fight but drops about 70% power. It has since become a bit puffy, so perhaps i over ran that one.Otherwise i use 3.3amp lipo. I have also used a 5s 2.2 amp zippy 40C battery, whjich lasted a 3 minute fight plus half another fight - ran out of time to swap. just depends on how you use it or your driving style.

There is a full calculator online somewhere, but dont know where - perhaps on askaaron?

for joints on big cables, one method - although not the best due to its high resistance, is to use eye crimp terminals and blot them together.
Another way is to use a junction box such as Wago connectors or chocolate blocks- although not designed for battle knocks and bumps as these can come loose. The best method is to use Y connectors using xt60 see here - or for space saving/weight, use a large watt soldering iron, strip the wire ends, hold together and wrap them with some copper wire. The copper wire helps get heat around the wires and solder them together, then add heatshrink over the join.

links as examples available. other stores/prices are about, i just chose a random link

https://wheelspinmodels.co.uk/i/2631...CABEgJJm_D_BwE