tJ, 19mm thick plate glass will be fine for a 6". Most larger glass shops can cut this as they make table tops with it. Plate glass ( not the tempered stuff ) is usually well annealled. The glass will be so cheap that I'd recommend the luxury of a nice glass tool to grind it on.
Get a 4" angle grinder with a $10 diamond blade and cut 6mm deep channels in the glass on a 1" grid pattern, and that will give you great abrasive flow and prevent the discs sticking together on finer powders.
If you can't get your abrasives and polishing powder locally in descent time I can give you the name of a place in Sydney that will supply.
I'd suggest at this time of the year you may have to be patient with your Adelaide supplier
