Jon, I was trying to illustrate price not practicality. As you know, there are a variety of tank sizes. I wouldn't go for the one I linked to as it's so small as to be worthless. I have the 2.5 gallon tank which fits easily under the 80. A 2 gallon would be even smarter fit. But if the OP is going for a fire extinguisher, they didn't say what size. A 9 L AFFF perhaps? Or a kitchen halon (banned grr nasty, trees die blah)
Having an accumulator to let the pump run on in between tyres is entirely sensible. I'd fit a pressure switch to that also so that the pump could tick in and out on demand.
The point here is that commercial tanks are not expensive especially compared to the buggery factor in trying to make one out an old extinguisher.
Unless you are a genius of course. Wihch I is nott.
Chris