5 ton should be about right for your shop. You have about 20000 cu/ft.
20 btu per square foot at 8' ceiling. Which you almost have double plus lower r factor. Estimate 3000 sqft.
X 20 = 60k btu=5ton
If it is similar to a mitsubishi, sanyo, fujitsu. You should have no problem changing the the blower wheel. Left side has a sealed bearing . Right side is attached with set screw to motor. A lot a screws in cover and sub cover. But I have replaced many without pumping system down.
With pump off. Here is a pic of proper hose, pump, gauge location. Notice the gauge is at the service valve with an isolation valve to let the gauge read only the hard piping.
Need to place gage at unit on core remover like this to get accurate reading.
http://www.climatedoctors.com/Shop-by-brand/Appion/Appion-MGAVCR-Megaflow-5-16-Valve-Core-Removal-Tool?utm_source=google&utm_medium=cse&gclid=CNmQnoyavr4CFYcDOgodTzIAfA