I have a 35X40 pole barn style building with r panel siding/roof, 3" batt insulation, about 10' average to the eaves, three insulated 10x7 doors, and an insulated man-door.
For heat, I found a 3.5 ton house-style AHU for $250 used (it's a fairly new model). Once it had run a couple times I realized the fancy shmanzy digital motor controller had died. Instead of spending $500 on a new motor or $350 on a new board, I bought a traditional blower motor, start relay, and cap for right at $100 from Granger and backdated the AHU. I had a plenum built for it, but never got around to installing it or running ducts. With a box fan in the rafter to push the air and a ceiling fan to push it back down it'll take my shop from below freezing to the 60's in a VERY short time. I spent around $80-$100 on wiring, conduit, breakers, and 2x4's to build a stand/intake plenum. It's getting harder to score old units from contractors, they scrap them for copper money FAST. Best to troll CL or give your name/number to a HVAC contractor you trust, so they can call you when they pull a decent unit out.
The shop came with a nice wood stove, but it's really impractical if I'm just out there for a couple hours. I have it for fun/emergencies, but with the electric AHU... walk out, flip switch, get working and in awhile "man it's pretty comfy in here".
For a/c, I want to add a compressor to the unit I described abve as is has an a-coil, however it's an R22 unit, so I'd need a new thermal expansion valve and I'd have to score a newer model compressor to run the new refridgerant (Puron, I can't remember the code at the moment), then call in a favor to have the lines run and charged. Nevermind that adding the two breakers for the AHU basically maxed out my panel, so I'd need a new sub-panel.
Currently I have a 15500 BTU window unit a co-worker gave me installed in the work area / lounge area of the shop. It's run on an isolated 20 amp circuit. It doesn't begin to cool the whole place down, but it takes the bite off for sure. I keep trolling CL looking for a used 220V window unit to install on the opposite side of the shop. I know running both will MURDER my electric bill, but I'm only out there in the evenings and weekends and having the ability to take it from 110deg in SW Oklahoma down into the 80's is worth every penny. If I own that much insulated/enclosed space, I should be able to at least make it comfy when I want.
I also bought a tiny 5000btu window unit for $40 from a buddy. It's installed in the corner I enclosed for my doghouse and runs to a switched outlet. If we have to be gone a long while or it gets stupid hot, I can switch it on and easily cool down my doghouse. I know it sounds ridiculous, but so does $28/day to board my dogs. I also have an automatic dog waterer in there, it's pretty plush
