BTUs are BTUs, weather it is a split unit or a window one. I am in south Texas, near the Gulf, and have a similar sized shop, aroung 22' x 22' with an 8' ceiling and a storage room, about 8' x 20' off one corner. My water heater is in the storage room so that adds some heat. It is partially insulated on the northern exterior wall. The west wall is almost all garage doors so not much room for insulation there, but if I replace them I will get insulated ones. The other two sides are common to the house. The ceiling is not insulated.
The thing about those on-line calculators is that they are for the average, living space in an average house. They do not take in all the factors. In my case, I tried 10,000 BTU and then 18,000 BTU units but both were somewhat lacking in the heat of a south Texas summer and for the extra heat load of the water heater and the machines being run in the shop. You must remember that any electric usage in the shop WILL become heat: 100% of it ultimately becomes heat unless it manages to escape out of a window or door. But here, in summer far more heat will come in those openings than what can manage to escape so 100% of that electric usage is a realistic and practical figure. And this adds to the amount of AC BTUs that you will need. Of course, in the winter it subtracts from the heat needed.
I presently have my second 22,000 or 24,000 BTU window unit. Both of them cooled the shop well, even in the summer. Both had heater coils that were sufficient for the winters here; I don't know about Tennessee. You may find that 18,000 BTUs are not enough on the hottest summer days. Or it may be.
My prior unit was a Frederich and the present one is a GE. Both of them are relatively quiet. In case you are curious, the Frederich failed due to clumsy maintenance on my part. I hope not to repeat that mistake.