Porta potty for a 9-month barn build: rent or buy?

Rent it on a long-term rental and have it serviced once a week. Buying looks cheaper until you notice the vacuum truck still has to drive those same 40 miles to pump your unit, so the trip doesn't go away. You just lose the cleaning and restocking that come with a rental service visit.

For four to six people, one standard unit covers you. OSHA 1926.51 calls for one toilet for 20 or fewer workers, so you're well inside that.

The bigger issue for you is winter. Nine months starting now outside Bozeman means that holding tank is going to freeze. Ask the rental outfit about winterization, usually a brine or antifreeze charge in the tank, and whether they keep servicing when your road gets rough. If you own the unit, keeping it from freezing solid is your problem.

We framed a pole barn over a winter and the owner had bought his own unit. He ended up scrubbing it himself every week and chasing down someone licensed to haul the waste. He told me he'd never do it again.

Park it on the gravel at the end of the driveway where the truck can back right up, stake it down against the wind, and you'll be fine.