You're going to want to have support a maximum of 24" apart; 16" would be better. In that case, you might as well just use conventional stick framing. On my barn, I went conventional framing, using 2X6's because of the wall height (12') and for more insulation potential. I see lots of people build a pole barn with posts 8' or more apart, then use more time and materials to fill in the area in between so they can hang siding, etc. I can't inagine it's much faster or cheaper in the long run.
Depending upon the siding, you'll also need bracing to keep the building from wracking in the wind. Sheathing it in OSB or plywood takes care of that; the siding is there for aesthetics and to keep the weather off of the sheathing. I sheathed my barn with 5/8" T1-11 siding to look like 8" boards. Someday I'd like to cover t with better looking siding. I did the house in Hardie plank, but it's expensive and labor-intensive. Materials alone for my barn would run $2500, not including labor.