Lack of ventilation is clearly causing your mold/mildew problem, so a functioning window will help, and the larger the opening the better.It does require a match on the exterior finish material to fill in the size difference, but these baths are always on the side or rear of the house, so it's not very noticeable.The fact that it's a different size and more contempo-looking than the older original windows, isn't a compromise on esthetics, either.

There about 100 million posts about “what should I do with X amount of money” so I’d recommend utilizing the search function to find those and read them.

You may want to partner with someone who's familiar with the marketing side of flipping, because this is the most important function.

The "popularity" is just  a function of how many gurus are trying to make money selling you the "secrets".

Unfortunately most detached duplexes are smaller homes (our largest is 4 BR/2 BA but most are 2 BR). 

-What is the smallest/ largest loan you make (to see if you can build a relationship and scale with this person)?

It may be that most of the largest syndicators only take accredited investors but there are many of us who are happy to work with non-accredited investors under rule 506(b). 

There's a function IRR in Excel that does this for you.  

I would plan to pay for majority of upgrades with a line of credit.I have a few additional reasons beyond monthly cash savings to sub-meter the electric: 1) There is one thermostat in the house, controlled only by tenants in the largest unit.

I was already leaning to the side of just stashing the money away so it will serve either/or function, first as a down payment for your next property or money to pay major capital repairs or improvements.