Dust free
I have the Delta unit in my garage since I do lots of woodworking and also park the cars. It helps, but it's not perfect. I think one of the other sources of dust comes from outside, through all the gaps.
If you want to keep all the outside dust out, I would suggest installing a fan that pulls air from the outside to the inside. Put a good filter on the outside! This will increase the pressure inside the garage and force air OUT through all the gaps.
You might think you can seal all the gaps, but it's actually pretty darn hard to do...
Good Luck!