Each building has a 4,000 sqft commercial ground floor.

Generally speaking, pre-fab homes come with existing HVAC and Electrical systems, flooring, insulation, etc. and some brands even promise they will meet the specs for the local building codes anywhere on the continental U.S. 

I had a floor supervisor tell me once that "If someone shows up on-time, sober, works hard and does not steal they are 80% of the way there"    I thought it was really cynical at the time.  

Originally they said I need a return air duct to get more air going to the second floor.

This probably the nicest duplex I've ran across in my market mainly because its newer, has nice upgrades, great floor plan, well built, and its in a great town.

The cost to remove them, redo the sub floor, new tubs, plumbing and tile work does not appeal to me.

First I decided that I wanted to update the kitchen and bathroom, replace the galvanized pipes and hot water heater, and add a half bathroom on the main floor.

i was thinking ceramic tile throughout, but my contractor recommended the vinyl wood-looking flooring.  

My original budget was 15,000, I will be about 30,000 all in after this week when my last tenant moves in.Here is why:3,000 in unknown structural repairs (termite damage)3,000 in unknown electrical repairs (they had KnT hidden in the walls upstairs, yet tied into new romex at the breaker box, and the outlets/fixtures, it was all replaced)3,000 stolen from my original contractor that I had to fire (lied about work being completed, redoing much of the work he was paid for/ did not actually do) 2,000 for a new AC just crapped out last weekI would say the other 4,000 or so was my own decision to replace a few appliances, add higher end flooring and fixtures, and just overages on my end. 

I can finish the floor and wall.