You're mad to ,start with Robot Wars whatsoever. Build something that takes a lot of time and efford, and then hope it won't get smashed in 5 seconds.
Join the assylum.

Hardox is a fine material for the spinner.
The grade, as long it's higher than the opponents armor, it's good. so aim for 500+. Valkiries disk is hardox 550, 12mm thick.

Expect everything to break sooner or later.

Brushless motors have mounting holes on 1 side.
The NTM outrunner has threaded holes on both sides. Mounting to the frame is done with the 4 M4 holes on the side with the wires coming out. The other side, on the spinning body, has 3 M3 holes to mount propellers to.
Oh, and ruggedize the NTM. The weak point are the bearings and the magnets. bearings can be replaced with decent brand bearings, and the magnets really love a lot of glue.

Hannibalito4 and Valkiri3 used a 25A per channel esc, and they moved fairly well; but with higher graded esc's it went better. the 30amps could work. But take the advice, go for the TZ85's.

Hardox, small bits of high grade Hardox I buy at a place that makes excavator buckets and simular.
Expensive, compared to buying a big sheet and cut from that, yes, but a full sheet goes a few £1000, and a 350mm diameter disk goes about £100 in raw material.(and around £150 in lasercutting cost)
The proven and renowed Hardox 450 3.2mm thick we buy in bulk with the DRG, and then we get a pasture sized steel plate of 2.5*6 meter (last time it was even a 2.5*8m), And then the cost is "less". If I recall correctly, divided in sheets of 1.5*1m it was about £160 a sheet.