$18 a sq ft seems a touch on the high side, but not rediculous. It's also a regional thing, so it might sound like allot to me where labor is cheap compared to other areas where labor and permitting are expensive. If it's a company with a good reputation and a history of quality workmanship, then that's all that really matters.
How far from your home and water source will the building be? Having water, or not having water, is something I would strongly think about. This building will be there for a very long time, and I think having water available at the building is one of those must haves. One spickit at the front near your big door and another inside. If you have permit issues with putting a sink inside the building because of no septic tank, then just add it later after the final inspection is done. For an inside sink, you just run a drain out to the bushes to water the plants. As for dealing with freezing temps and not being there, it's done all the time with weekend cabins and other rarely used buildings. Put a good quality gate valve in the ground where you can easily get to it year round. I've seen them outside of buildings, but don't care for that as it's a pain to turn on and off if it's raining out side or snow on the ground. Putting it inside the building keeps it safe and easy to turn on and off.
Eddie