Wish I could help you with your area specific questions. The next time you are "back home" call the City or County planning department and ask them your questions. Three years is time enough for things to change so touch base with them again before you start.
I retired 15 years ago (40 years of combined Army/Civil Service) and the first thing I did was build a house for my long suffering wife. Then I built my shop.
As an observation, the foundation and the site prep are the two things you can not go back to and fix once your structure is up. Don't cut corners. If you need to bring in fill then bring it in. Don't get miserly on rebar, concrete, or mix strength. If you have a drainage issue going in then fix it before you build. It will get worse when you add runoff from the roof.
If you decide to use the old slab don't build on it, build around it. Just because it is already there doesn't mean it, and particularly the edges. are adequate to support the load imposed by the vertical construction.
Before you commit to incorporating the old slab in your construction plan determine how flat it is. Just because it looks flat doesn't mean it is flat. A little slope to run water/ice melt out the door can be beneficial but you don't want it puddling in a back corner.
Since I don't know what kind of "work" you plan to do in your shop so it is hard to make recommendations on your overall plan but as an observation, 16" is not very wide. It may work for you but I think I would shoot for at least 20".
Good luck.
Garryowen Sir!!