I have a slide-in ac unit with electric strip for heat. I have been using one (on #3) for almost 20 years in my 16x20 shop. I keep the stock filter clean and I also built a box over the face that houses a pleated filter that I clean and change often. So far, so good. I will note that the first time I use it each season, I do get a faint whiff of burnt wood smell. Oh, I also clean the unit at least once a year inside and out. I also blow the vac the coils that are behind the filter now and then. Anyway, that has worked for me.
Tony - almost time to take the unit out and give it a bath in time for the warm weather...if it ever gets here!