Software often promises great results with little effort - beware!
All software has a learning curve. Ask yourself if you want to be designing a number of items on a ongoing basis or just this shop. If the number one task is to get the shop building permit and built, have you considered paying someone to make these simple drawing(s) for you?
Your lumberyard may know someone who would do a small project like that.
If you are going to learn to use a drawing software, I would definitely recommend SketchUp, a proven mainstream platform, with serious capability and tons of support and tutorials on YouTube including Ron Paulk videos.
Puchased from Google by TRIMBLE some years ago, SketchUp "Make" 2016 version is free for non-commercial use and you can try it without risk (except for your time - it's addictive).
Let us know what you chose to do and how it worked out:)
