Another thing to keep in mind is that it's worth leaving some wiggle room inside the bot. Your outer dimensions sounds fine, but when you come to pack everything in you might struggle with wiring and stuff like that if you pack it in too tightly (this is certainly a mistake I made!).

Aluminium is not a great material for taking impacts. It's not awful but HDPE is much better. Keep in mind that HDPE is about 1g/cm³ vs 2.7 for aluminium, so for the same sized panels you could either have 4mm thick Al or 10mm thick HDPE and the weight is about the same. The HDPE version is not as strong or stiff but will take impacts far better - the aluminium will tear, dent, pull out from its screw holes, etc, whereas HDPE will get gouged but only "locally". Plus, HDPE is much easier to work with.

All that said - if it's held together right, a 4mm aluminium shell isn't going to disintegrate in a stiff breeze or anything.