Mini-splits are really efficient, but they work best in super-insulated environments. The initial cost difference between continuously insulated advanced framing and a conventional 2x4 wall is minimal. The savings can be huge, especially in a more expensive energy market.
If this is an attached garage, I would simply tap a duct in my house and run a duct to my enclosure. You are correct mini splits are efficient, expensive up front, require a permit to install in any jurisdiction in the US, I know this without any doubt. Also requires it’s own circuit. Would also require an inspection by a mechanical and electrical inspector, this could be troublesome, especially if you are in a state where it is not yet legal to grow and could cause trouble down the road. You could probably get away with building the enclosure in the garage pretty easily. If you ever need to have mini split serviced, service tech could cause issues. I’m just saying, even if I lived where it was legit, I still wouldn’t want anyone knowing my business. If it were me, I would keep it simple. I would frame the walls with 2x4’s 2’ on center, double the studs at door frame and first stud bay on each side of door 16" on center. Drywall interior and tape and block coat only. Buy a bunch of emergency sleeping bags to cover interior surface. Insulate with R13 or R15. Wrap exterior with poly and staple to studs. Tap duct work in house. Tap electric in garage. Get a couple grows under my belt and by then I would have figured out any upgrades that I wanted. Exterior of walls would still be open and easily accessible to run new wiring or whatever and then recover with poly. Just my $0.02 worth but, what do I know. I like to keep it simple and not try to overthink it in the beginning or you may talk yourself of even attempting it. Again just my 2¢ worth. Have a great day
“It’s for wine storage”
Exactly. I’m actually a master combination code official where I live. I definitely know how to beat them at their own game