I bought these and have been really happy with them.
For just pistols you could probably get away with 3/8 or even 1/4 inch but the thicker plates will standand up to 45gr 223 and surplus Mosin Nagant loads from 40 yards in my experience.
I welded up a frame out of junk rebar and short lengths of scrap chains to hang the plates from. I used 3/8" carriage bolts with 1/2" washers to attach the plates to the chains. One thing I would change is make the chains a few links shorter because the smaller plates will wrap around the top bar.
Attachment 256877