Adding a washer/dryer for studio to use too.

You can make minor adjustments based on the type of appliances (dishwasher, clothes washer and dryer, air conditioning, etc.) and the size of the rental.

Each side of the duplex has 3 bedrooms, 2.5 bathrooms, washer/dryer, and dishwasher.Each unit has a family of 5 living in it.Would it make more sense to replace the hot water heater with a tank or a tankless setup?

I don't know if this makes it easier or more complicated, but our upper level may not have any gas services at all (husband is going to the property tomorrow to see if upper water heater is gas or electric, and whether washer/dryer in each unit are gas or electric).

New modern lights and ceiling fans, quality furniture, Big screen TVs (not so expensive anymore), big washer and dryer, one of those cool refrigerators.If you ever go stay in a nice hotel or nice airbnbs, try to pick up on some of the things they do to make it feel luxurious. 

And then in comes the junk....we're now at 20 cars, a bus, an RV or two, a tow truck, washers/dryers, and even an airplane fuselage...yes you read that right.A bunch of us neighbors called the County of course and they sent out a Code Enforcement officer.

Keep the cute house neat inside and out.Annoy thing: since she moved in, she asked to replace dryer, then dish washer(said after washing will have water spot on dishes), then clogged under kitchen sink after Thanksgiving, add light... finally we had a quite holiday.   

It doesn't have a washer or dryer, but I've been thinking about placing one so I can attract more tenants.

The other issue to think about is mechanicals i.e. heating and cooling access and control, water heater access, and washer and dryer access. 

We installed LVP throughout the unit, added in unit washer/dryer for each units, completely redid the kitchen, and minor cleanups to the bathroom.