Most people who start with robot combat in "our format" do that by building one of the lighter classes. Those are a tad lighter on the wallet. But it's not up to me to decide that.

For armor. Most used atm is wear resistant steel, like Hardox. And simular kinds from other steel manufactorers. Search on names like RAEX , XAR or Creusabro. A tad more expensive than normal steel, but a lot cheaper than anything else for the same value of protection and "ease" of working with it.

The really high end grades of ali can do the job on the same level, but that you'll feel in the wallet. 7075T6 springs to mind.

Some plastics have , if used well, a reasonable result. In the past, polycarbonate -brand names like Lexan of Macrolon- was popular, but is getting replaced in most cases with the lighter, toughter cheaper, but softer HDPE.

On the other side, we have seen robots made from mild steel, ali checkerplate or low end stainless steel performing very well in the arena, and machines armored with hugely expensive stuff fail miserably. It's all how you use it.

And remember. Air is good armor, but armoring air is rather futile.