Now, he was never been in the duplex, which has wood floors, brand new kitchen cabinets, metal accent wall and all new lighting and painting etc.

Tiled kitchen backsplash? 

Assuming the plumbing is fully charged, and the home has been inhabited recently, you can use the moisture meter to probe around areas that are common to damage: plumbing walls, cabinet toe kicks and bases, kitchen and bathroom floors, shower or tub surrounds, below the washer drain box, etc.

But I now run food grade anti freeze through the lines as I added 2.5 baths & a 23x12 kitchen all with extensive Pex plumbing.

Personally I think there would be buyers that would prefer a more modern/rustic style with new kitchen and bath, etc. 

If you are doing basic updates like paint, cabinets, kitchen etc...           

Each unit needs new paint, flooring, kitchen hardware, appliances.

We kept the same cabinets in the kitchen and used a staining trick that has saved us plenty of money up to this point.

I plan to reconfigure the kitchen and add a 1/2 bath on the first floor, as well as modify the full bath on the second floor.  

Does it have a granite kitchen with SS appliances or a 30 y/o laminate countertop with dated appliances?