Open ended question time.
First, a few caveats.
I can only get SAE/Imperial sized stock, i.e. 1/4", 3/8", etc., and in 304 grade. That'd be 6.35mm and 9.525mm.
I am very much a self-taught welder, always improving.
I'm set up for MIG and TIG process, far better with a MIG gun than a TIG torch...