Have you thought about a HVAC rooftop unit and just mount it outside ground level? Our 30x45 has a 93,000 BTU (heat) unit not sure on the cooling tonnage, in the winter it kept the shop at 68-70 and in the summer it's keeping it at 72~.
We have a single vent in the warehouse and then a duct to the office. I see them pop up on craigslist for less than $300. I am planning on picking up another as we are going to be adding onto the shop this fall.
It works great for keeping the humidity down as we work with metal, when we don't run it things like to flash rust.
Its also nice to have the air filtered, keeps down on the dust. Though we are going through a filter change per day...
Another thing to think about since you have such high ceilings is to put a few ceiling fans up, our old shop had that and just having the moving air helped a bunch.