Metal selection 101

Hi All,
Here in Phoenix we have a good surplus metal place called 'Davis
Metals.
Lately I've been wandering around trying to pick out useful steel to
machine.

My goal is to find a steel that is easy to machine and can be sent
out to be hardened if needed. This will be for general purpose
toolholder/vises etc
Any suggestions?

What field tests can be done to determine if a piece of scrap steel
may be useful for machining? Tim mentioned the spark/grinding test,
and I was also thinking of a spring loaded punch and battery powered
drill. Are there tips from the
experience pool on this...

Thanks,
Lee