Rules of thumb for sizing cooling loads as used for residential or retail/office type commercial buildings are useless for sizing equipment to cool a machine shop.
Every watt that passes thru your meter is ultimately converted to heat. Doesn't matter whether that watt is powering a light bulb, a spindle motor, a coolant pump, a hydraulic pump, or a personal computer, its all converted to heat.
If you had an average load equal to 20hp all day long, that's 15kW neglecting inefficiencies. 15kW X 3413 BTU/kW = 51,200 BTU/hr, or 4 1/4 tons of cooling just to offset that load. Add in the various inefficiencies, all of which are also converted to heat, and you're likely pushing 5 tons.
A competent contractor can run a heating and cooling load on any building you may buy or build, but to have accurate numbers on the internal loads, you'd need to collect some information on your own. The last thing you want is someone to walk thru and add up the horsepower of every machine in the shop and base the equipment selection on that number as it would result in a grossly oversized system. That said, if the contractor has nothing else to go on, he'd tend to oversize to cover himself rather than take a chance on undersizing and risking a civil action later for installing a system that won't cool.
You can get your load information now, as it would normally be independent of the location of the business. Its as simple as using a clamp on amp meter to check and record your incoming amperage once per hour during work time for a couple weeks. Also, keep a daily record of how much total coolant mix you have to add during that same 2 weeks. The coolant amounts will give whoever's doing the load calcs a good idea of the latent (humidity) load, as that can be a far greater percentage than one will normally encounter in most other commercial buildings.
When you give the load numbers to anyone who's doing the overall load calcs, make sure they understand that your numbers are for internal loads alone and do not include the normal building load calculations.
I'd stay away from even thinking about using mini-splits (the precharged units you mentioned) due to the fact that coolant mist will gunk up the evaporator coil on those units in no time and make a maintenance nightmare that will cost you more than you'd save via their high efficiency. Mini-splits are fine for use in a garage or similar setting, and would be okay in a machine shop if all the cutting was done dry, but that's not the normal shop situation.
Personally, I wouldn't consider any bare concrete block building in your area unless you're willing to spend a bunch of money on insulation. We rented a 4000 sq ft building with 16" thick poured concrete walls for a few years prior to building our shop in the late 80's. Those walls have roughly the same R value as 8" block walls. In the coldest months, we would burn thru a thousand bucks worth of natural gas per month and the place was still about like working in a tomb. In comparison, our current building is a 9000 sq ft pre engineered metal building with a gas furnace for the office area and vacuum tube radiant heat for the shop, and in the coldest part of winter the gas bill never runs more than $400 while the working conditions are very comfortable. And FWIW, the winter design temperature here is 20 degrees higher than in your area.