A couple of thoughts. Regarding insulation. I think that is key as this better allows an AC unit to keep up with very high heat and saves utility $. I think reading about R values is a good use of time. If the framing is 2x4s, consider adding some strips to have a larger cavity between the studs to accommodate more insulation. That could be R-Max rigid foam (available Home Depot with high R-value per inch) or the other less expensive easier to install options.I think split is better than window which is better than portable. A central can be use for larger structures where the air handler is suspended and really, there does not have to be much ducting or any ducting. That would give maximum cooling. If you like working in the heat, not a problem. If not, then it will be no fun when the inside temp is 80+ degrees.
Another thought is that if the structure has windows and you can remove a window and build an enclosure for the dust collector outside the opening where the opening allows the duct to exit (and air return), an advantage could be an ability to empty the bin without a dust cloud inside the shop and that space saved inside the shop can be used for other things.
Of course all about priorities and personal preferences.