"...I don't think a more detailed analysis is needed..."
Think again, or you'll spend way too much, and end up with another oversized less-than satisfactory solution. The time to do the analysis is NOW, before you've signed contracts on heating systems and windows, etc.
The thickness of the brick matters for assigning a U-factor. If it's a 12" thick triple-wythe (with no ventilation cavity behind the outer wythe) you're looking at about U0.42. If it's 12" thick cavity wall it's more like U0.32. If it's thicker than that it's even lower.
New windows have a payback time on energy savings somewhere between 50 years and never. Drafty single-panes that can be re-weatherstripped & tightened up are worth repairing, then installing
exterior low-E storm windows over. Hard-coat low-E glazing adds upfront cost, but cuts the payback period in half due to the higher performance. Typical payback in residential applications is on the order of five years, but in a building that's only brought fully up to temp on weekends it'll be more like ten. An exterior low-E storm over a reworked wood-sash single pane double hung performs at about U0.33-U0.36, and is usually less than half the cost of an equivalent-performance replacement window. Low-E storms can be either fixed or operable, but the fixed windows will always be air-tight. There are multiple vendors- if you can't find any through local window contractors, Larson distributes through the big box store home-center chains.
Twelve tons of compressor is an insanely high overestimate of what this place likely needs to cover the load, even if it's a crappy one that falls off a capacity cliff at 25F. Assuming it's only delivering 6000 BTU/ton of compressor @ 0F (instead of 11,000-15000/ton like a high-efficiency mini-split) that's still 72,000BTU/hr, or about 25 BTU per square foot. That might be realistic after tightening up the windows and adding storms.
Assuming you have something like 12' walls, and the common-wall to the other space is the 43' and a very bright 25% window/floor ratio, concentrated on the 63' walls, that' means you have ~650 square feet of U1 window, and about 1000' of U0.4 wall (worst-casing it), and ~2900' of ~U0.4 roof (assuming 2x planking and a couple layers of shingles.) So at a 70F delta that's:
Windows: U1 x 70F x 650'= 45,500 BTU/hr
Walls: U0.4 x 70F x 1000'= 28,000 BTU/hr
Roof: U0.4 x 70F x 2900' = 81,200 BTU/hr
Add it up and it's 154,700 BTU/hr, which is probably how they come up with 10-12 tons. At 35F outside/70F inside it's half that.
With low-E storms the window losses drop to ~16K...
...the next time it needs re-roofing, putting a layer of 2.5" nailbase EPS (panels of
half-inch OSB laminated to 2" of EPS insulation- there are many vendors) under the shingles would cut the roof losses to 18K...
...and your total load would be on the order of 62K, cutting your heating bill by more than half.
Even 4" nailbase roof insulation might still be cost effective, which would cut the total roof losses down to the 12K range and would reduce ice-damming/icicle issues by quite a bit. Talk to a couple of commercial roofing contractors, and get budgetary quotes at different thicknesses.
It's worth making a plan, keeping the functional gas-burners going as you execute the plan, but size the heat pump system only for the "after" picture rather than for the load on the building in it's current state, then apply the savings on the mechanical systems to things like storm windows and insulation. If you do it in reverse order you end up spending too much up front, and the systems will run less efficiently.