If you use your tools frequently, you know where they are and labels are not needed. If you are an occasional user or have a show place garage you need them so you can find them.
You can see in the picture the tell line where the slab used to be. It looks like it has slowly been sinking over the years. This is just poor construction by the builder. I have seen this done dozens of times. When the foundation of a house is built, the porch area is typically left empty...
Bill you may want to consider a subfloor something like this:
https://www.homedepot.com/p/DRICORE-Subfloor-Membrane-Panel-3-4-in-x-2-ft-x-2-ft-Oriented-Strand-Board-FG10006/202268752
There are other products designed for basement applications that will work also.
Use a natural color exterior stain in semi transparent. Sherwin Williams has them. You could use a solid color exterior stain but it wouldn't give you the natural look your after.
As MrBill said above, get them professionally cleaned. They will send a brush through each duct and **** out the dirt, clean the grills, and then ozone the ducts. They will also clean the furnace plenum and change filters. It's worth the cost.
You don't need trusses. Stick build your roof with rafters and a few collar ties similar to post #17 or like this, http://www.whynotsail.com/shed.html . Put in a ridge vent, https://www.lowes.com/pd/GAF-Cobra-Ridge-Vent-3-13-7-in-x-48-in-Black-Plastic-Stick-Roof-Ridge-Vent/50353098 or a couple...
A flitch plate will work just as well as lvl, maybe better. I have done many garage door headers with a flitch plate with no problems. The OP's plan is a good one.