I see mobile homes, and steel garages, and large steel girder buildings, but nothing that is something i can put together and finish myself, with floor joists like this one https://www.alibaba.com/produc... 

The floor needs a cleaning -- but next - should I sand the floors and then put a varnish on them?

In many cases, the floor between the two is a conventional wood floor- i.e. wood joists, subfloor, and floor with carpet or tile as a walking surface.

The water leak was causing the flooring to warp.

@Joseph Chacko vellukunnel FYI if these are older mobile homes with the original roofs you will have floor issues, yes floor issues.

The first-floor unit is ~800SQFT and duplexed down to a finished storage area.

I've found that Rentometer either overestimates or underestimates the rent depending on the location and condition of the property (e.g a house with updated kitchen, bathrooms and flooring will have a higher rent than a house where everything is 15 to 20 years old but still functional). 

We came in painted and replaced all the flooring with Mohawk LVP flooring.

To add the unit you will most likely need to achieve proper fire rating between shared walls/floors/ceilings and have interconnected smoke detectors.

Well here's what we ran into, David:You by and large need white trim if you're going to do cheap vinyl replacement windows, any sort of easy quarter-round on the floor.