I have been a building contractor for almost 34 years (only 16 to go). I would never go with a Pole barn, I have replaced several rotten posts over the years on buildings that are 20-30 years old. Especially now with the new treating process, it’s no where’s near as good as the old treating process.
I also agree that a steel roof will leak. I’ve had several disagreements with other builders over the years on steel roofs and I can show you several pictures of roofs I have replaced because the screws don’t hold anymore. Steel expands and contracts with temperature change (A LOT) When this happens it pulls on the screws and lifts them loose. That being said, a standing seam metal roof is a good product but I don’t like the way the snow slides off and destroys decks, shrubs or anything in its way. My recommendation is to go with a 12” Grade beam thick edge slab, foam under the slab, foundation and perimeter. Also put a 6 mil poly under the slab. Then frame your walls 2x6, 2’ on center, then a good quality house wrap. Depending what the surrounding buildings look like I would either put 1x4 wall girts 30 inches on center, and then vertical steel with exposed fasteners. Or sheeting and siding. Put the roof trusses 2’ on center directly above the studs. Then 5/8” OSB roof sheeting, 2 rows ice & water shield then synthetic felt. Good quality shingle such as Certain Teed Landmark’s. Make sure you have a 24” overhang on the eaves and a minimum of 12” on the gable. Vent it properly and have it insulated. I prefer the Blown-In- Bib system. You can get an R- 23 out of 2 x 6 wall. I have built many sheds including my own shop this way and the only upgrade I would recommend if you could afford it is to go with 4’ frost walls instead of a thick edge slab along with in-floor heat.
Todd B