Most of the population moved to the surrounding suburbs or other cities and abandoned the downtown area, causing it to become a center for poverty, drugs, homelessness, and crime.

TIAErich  Erich - I would echo other's comments, but also point you in the direction of a Bigger Pocket book, "Estimating Rehab Costs" - It's a 2019 version, so the actual dollar amounts will need to be increased, but it can give you a good starting point for the process that you can refine/update over time. 

You say you are new to flipping so maybe consider teaming up with someone more experienced for your first flip to increase your success rate?  

The fact someone wanted to show a 20% versus a 15% return by using a lower interest rate, meant increased risk, which you either win or lose, since you're gambling.2. 

Our battery went out and our son had to crawl through a basement window but I doubt too many people could have done that, he was impressed by our increased security but didn't want to drive 2 hours back home so he found a way.  

I’ve personally house hacked in the Irving park neighborhood in Chicago with a multifamily so not completely relatable experience to what you’re looking to do but hopefully can provide a few insights.If you go with the suburbs in a SFH, you could rent by the room and presumably increase cash flow.

either by increasing direct profits or creating growth or both.

A standalone ADU is almost always going to cost more than doing an addition, so the value added or cash flow increases will have to justify each.I work with a team of investors who will buy SFR homes then build a new ADU on the lot and add a separate drive way and meters for utilities in North Texas. 

They will try to cut the FED prime rate, increase tariffs, and shake up global trade.

I have a feeling the snowball of moment is increasing, and you will have that 4 plex in no time at all!