+1 on spray foam. That would be about the only reasonable way to insulate one.
I just acquired a 25'wide by 36' long quonset building for free fiddy. It was my father in laws and he had bought it from someone else that never erected it. It has one endwall in the kit, but the panels were getting some rusty spots on them, I think they'll still be fine though especially if the inside is sprayed with insulation and a guy could coat the outside with some of that white roof coating... I was told it was a 25'x 40, but there were 76 of the big arches and 18 small pieces so I figure one of the small pieces got lost over the years, and a few of the arched pieces that were on the bottom of the stack had some damage. I will just use the 18 small arch pieces and 72 of the large arch pieces to make it 36' long and that way I will not have to use a couple of the damaged arch sections.
My boy and I just spent basically the entire day taking that stack of parts apart, scrubbing them down with a stiff bristle brush to get all the dirt off, and then re-stacking them inside a shipping container to keep them out of the weather so they will be good to go whenever I decide to put it up. The endwall pieces are a lighter gauge than the arches and like I said had some rust damage. We scrubbed them clean and then coated any rusty spots with some WD-40 before re-stacking them inside the shipping container.
Oh and it didn't come with any bolts, so I'll have to source those.