How much metal to get?

[MrForbes]

You can buy a bunch of metal now, but you really ought to wait until you know what need....especially for stuff like 1" square tubing that you can buy at your local hardware store.

I can't think of many uses for 1/4" aluminum, but I can for 1/16" thru 1/8", but mostly because we have access to some machinery that can be used to make it into neat shapes.

Again, I suggest you wait on ordering metal until you make at least a preliminary design for the robot, unless you have unlimited funds and can afford to buy stuff you won't use.