i used this website to calculate the weight of the the Armour for my 4wd that got 8mm steel round the sides but i had to use 3mm on the base and aluminium for the top plate to keep it in weight, and i just weighed the wheels motors etc and its about 12kg without the wepons

http://www.matweb.com/tools/weightcalculator.aspx

it was drill powered not speed 900