first floor garage mini split sizing

[Uofime]

Yet another mr cool sizing question, but I want to get it right and my situation is somewhat unique.

I'm in Charleston SC. I have an elevated home, which means my garage is my first floor.
The garage is about 750 square feet(which includes a staircase), the ceiling is 8ft. As I mentioned the ceiling is the floor of my house, is insulated and temperature controlled above.

Roughly scale picture:

There is a single car garage door which is insulated.
The walls are largely sealed and "insulated". Insulation being 2 layers of plastic sheeting separated by 3.5"

I don't expect the space to be 70 when it is 100 out or when it is freezing out. 78 in the summer and 55 in the winter is fine.

Calculator suggests an 18k, but my 2k sqft house only has a 2 ton unit so will I be fine with a 12k unit or do I need to spend the extra 400$?

Thanks for your advice

 

[pcmeiners]

Two efficient 9k units. Also if one dies it is not an emergency situation, and your tools will not get condensation, (2) 9k units are more efficient than an 18k unit.

 

[Uofime]

Two efficient 9k units. Also if one dies it is not an emergency situation, and your tools will not get condensation, (2) 9k units are more efficient than an 18k unit.

What do you like for 9k units?
Mr Cool doesn't seem to have gen 4 9k units available (or i'm looking in the wrong places). the SEER bump to gen 4 is nice.

I've done automotive HVAC, I can get the tools to do r410 if it makes sense.

 

[pcmeiners]

I am using Fujitsu units as in the link below. I went overboard and got the highest efficiency, (2) 12k and (4) 9k units. If you can make a plumbing flare,
follow instructions, and have the correct tools you can install a minisplit.
Mitsubishi also makes good minisplits, but not as efficient as the linked units.
If you skimp on pressure testing, nitrogen purging, evacuation and micron testing it could cost unexpected expenses, You can easily buy all the needed tools and still save a lot of money.

https://www.goodlifehvac.ca/images/Fujitsu_LZ_Series.pdf