My 2 pence worth

Build it around what you know.

Renforce all joints. (eg. the right angle bracket.)

Bolt armour to the frame. (use counter sunk heads wear possible)

put edging strip along the edges of the armour. (this makes a big differnce, more so with bolt-on armour)

Get the best motors you can aford.

If you are making a spinner weapon make sure the blade is balanced.

Make the armour thicker wear you think the most damage is going to happen.

For structural design use box, I, T, L or tube section as the strenght of the rod is in the surface not the core (though is section walls are to thin it can be deform and thus loss its strength).

If you are going to use steel try and see if you can get it case harden.
If you have access to flame weilding gear, use a mild steel or one with a higher carbon content, a 02 (oxerdizing) flame torch heat untill the surface is cherry red then cover with water (if it is a large area then you will have to have some pour the water behind the torch as you go a long)
PS. make the case small if its to big it will get ripped to peices