So, an update! (mainly maths)

I got 2x 250x250x8mm HDPE to use for spare material and gauge how heavy a bot made out of HDPE weighs. I came up with these dimensions and weights: Warning lots of numbers

Robot basic wedge shape 500x600x70mm with cutoff for wedge at the front
Base armour: 300x600x16mm 2742g
Ceiling armour: 300x400x8mm 914g
Side armour: 2x 70x500x16mm 1066g (wedge shape averages out 70x600 and 70x400)
Inner panels for central motors/electronics: 2x 70x500x8mm 533g
Front wedge flat panel 212x500x8mm 807g (trigonometry base 200mm height 70mm)
Rear armour: 70x500x16mm 533g
HDPE armour total 6595g (almost 1/2 the weight!)

I won't bore you all with the rest, but I have the motors and Saturn 16 gearbox weight, then I estimated the rest. Without a weapon, the robot will weigh ~9kg with the above armour. With a lifter, thats another motor, scoop, and gearbox/gears which will likely add 1 to 2kg since it's a small scoop at the front. Looks like plenty of margin for reinforcing armour or adding more inner panels.

I will add some pictures later of the parts I have so far! The radio controller looks awesome and it's the first time I have held one in the flesh.