That seat appears to be nothing more than a universal-type replacement seat for a riding lawnmower.
I'm guessing since you posted an .au link that you don't have access to the same stores around me here in the States, but I am POSITIVE that I know of a place where a seat similar to, if not identical to that one can be picked up for ~$30-40.
If you do a quick search on
Toro's website, I'm sure you'll come up with some SUPER PHAT "racing seats" that, while pricey, I can vouch for their comfort....you may just want the cheap version, but you'll see what I'm talking about - specifically, check out the Toro Z commercial series mowers - Z-560 is what I drive, give 'er a looksee.
2" PVC will prolly be strong enough, if you're wary of it, jump up to 3". If you're REALLY wary, move on over to black pipe, I guarantee THAT will work! (the metal stuff, not black PVC pipe)