30-36000 BTU sounds like an overkill for a well insulated 1200 sq ft shop, I think you'll be fine with 24000. I'm a bit south of you in SW Indiana. 2200 sq ft shop w/ 12' ceilings and a 24000 BTU mini split for the bulk of the shop, and a 9000 BTU for the office part of the shop (about 140 sq ft). I generally cool my shop to only 80 degrees but the 24000 works well. Wood in the winter for supplemental heat, as you know the mini-splits start getting costly when the temps get down below freezing.
Not looking forward to the return of hot weather next week, this cool spell has been great for outdoor work. Wouldn't mind seeing a few pics of the homebuilt. No skin means easier to see details.