Couple of thoughts on your two links for cutting: The Rage looks good, but you are dependant on the one blade. Any damage and it's a new one. The grinder would do the trick if mounted in a hinged frame on the bench. Discs are cheap, but as they wear, the depth of cut reduces. I used one for many years and one can always take it off the bench for freehand cutting or grinding, so a bit more versatile.
For the bench, going up to 30 or 35x2mm would make it more solid.
I think perhaps you would be better practising the safe useage of a smaller grinder before moving onto a 9" version.
A 4.5-5" grinder with a suitable disc would cover most requirements I would imagine.
safe usage? im probably just buying cheap discs, ill get some decent ones from weldquip, to be honest they have only even broken when cutting thick steel.
I just wanted a more practicle way of cutting through the box section