I completed a similar rehab: needed insertion of beams for structural support when we took down a wall between dining and kitchen, plus new deck, and a few window enlargements.  

I've found a property on the mls that is a 4/2 on pier and beam in a mostly mobile home rural area.

totally  agree with Neil Henderson tub  but with shower  doors they go for like 138 at lowes and the sorround goes for like 100 it's the Avondale  5 piece sorround it's not the prettiest  but will keep water in tub I hate water damage it's the worst before you  know it subfloor is rotten ( pier and beam house or upstairs)

It seems like a weird place to have a closet so the builder may have put it there because he needed a load bearing beam and this was a way to hide it.

Just observed that my wood supports on the foundation piers were constructed off center on the pier (about 1 1/2” off center). This is an older home built in 1964. Should I be concerned or was this a practice in older...

If you are moving load-bearing walls and adding beams you might need an architect and engineer, while if it is as simple as some updated finishes and casework you could get away with less sophisticated designers.

The older homes on pier and beam are closer in and very expensive so flippers can’t afford to not get those right, the ones that aren’t torn down.

Is it pier and beam or on a slab?

The main house is on pier and beam and is a 3 bedroom 1 bathroom. 

I could see the fungus in the black mold and the new lighter color fungus spreading down the beams.