(all 6-7 ft wide) Should I leave the insides empty for the new homeowners to put in their own closet system of their choice, or should I install a basic rack system like in the below picture?

The smoke detectors needed to be installed.

If you need a Storm drain installed or Office walls torn down and replaced, fewer contractors and you have to fit their schedule since you're a small player.

Is it possible to install new sub-flooring to level out the floor to use what you have?

I just got a quote for about $3500, labor and materials, to remove the old oil furnace and install the new gas furnace.Sounds like you already have the gas service, so that will cut out significant lead time.

I want to ensure that the door matches and that it gets painted, and also that it was installed professionally.

I could install carpet but there are beautiful hardwood floors and the second floor tenant would hate it with the mess of kids.

When laying the new pipeline infrastructure, install valves and futures to make the tie-in to public sewage easy once the township lays the lines.

Laminate and Vinyl Plank installation should cost roughtly the same.

When you drill into in installing vinyl siding it becomes disturbed and harmful.