I've had a GE Zoneline through-the wall unit in my shop for 18 years and the only trouble I've had required me to replace a buzzing power relay and a fan bearing...easy job both.
My unit is Model AZC906DAV1, but they've changed model numbers since I bought it. The equivalent today is in the "Deluxe 3800 series" and I'm sure it's more efficient, but mine delivers 14000 BTU, runs on 220V., pulling 8.5 Amps. These units are basically heat-pumps so you get cooling in summer, heat in winter and I can hold the shop at 70 +or-a degree or two year round. The heat-pump handles outside temps down to about 25 degrees..it cuts in the heat strips after that. The reverse pump air conditioning is great.
My shop is about 24 X 36 with a 9 ft. ceiling and I know it's well insulated because I built it myself. You need to put in some kind of air circulation or it will cycle on and off and overshoot/undershoot temp settings. I put in two 40" fans rotating in the updraft direction and that solved the problem.
The GE units are extremely quiet and very reliable (can you tell I used to work for GE). ..Plus the GE Customer Service for this product is great. They told me how to fix both problems and had the parts in stock...even now, 18 years later. I paid about $800 for it back then and they're probably about $1200 now. Most of these motel units slide into a standard 16"X42" through the wall box..supplied with them.
Hope this helps,
Don:gar-Bi