A window unit is totally a viable option. Basically cut a hole in the wall and you can have heat and air in a half a day.

Mini splits can be bought on eBay. I bought mine 2 or 3 years ago and has been working flawlessly since. It was $550. It took me a full day to install, it was my first time installing a split system.

I had a real struggle trying to get my flares right, I didn’t have the correct flaring tool. I ended up brazing the line. Looking back I should have left the line in tact and just installed it. I wanted it to be super tidy so I went through the extra effort.

Don’t over think it. If you plan on being there another year or so I would definitely do it. Insulation and drywall will make your garage more like living space than, well, a garage.

I’d show you a picture of my garage but this forum is from the 90’s and can’t handle anything bigger than a thumbnail.

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Eugene

10 E63 AMG
93 300te 4matic
07 BMW X3
14 Ford F-150 Fx2