I can do 90% of remodeling/repairs/plumbing/electrical work myself unless I’m away on business.

Quality work will go a long way.To give an example, an investor I know just bought a house that needed a layout restructure (basically redoing the layout of the house to make it more functional and attractive), installing central HVAC, building out a washer/dryer room, plumbing (new bathroom and kitchen location | rough in + top out), full electrical rewire (after full demo to studs), new sheetrock, paint, flooring, kitchen/bath finish out, and exterior restoration.

I would also tell him that you would like to have an annual property inspection report, which should include a plumbing, electrical and HVAC inspection.  

But then after that, you would need to buy a single-family home and work with a realtor and ask them to help you look for properties that have the potential to be converted to a duplex - which can be challenging, because their inherent structures themselves might not be suited for what you're trying to do (especially if you're trying to add a bathroom or separate heating, or plumbing units. 

If you do make an offer hire roofing and plumbing professionals to make sure there are no issues. 

There is probably old wiring, cast iron plumbing, clay sewer lines and on and on.

Renovated the whole house.... floors, ceilings, walls, plumbing, electrical.... learned a lotWhat was the outcome?

We also ripped out the walls to update 1930's plumbing, electric and R2 insulation. 

They found a bunch of issues with the home, after the electrical fire they had to require the home, major plumbing and structural issues were found!

Probably paint and carpet on the inside of most units, planning for significant capex on 1-2 (HVAC, plumbing, misc) Open to all thoughts and comments.