Main things I obviously am looking to figure out are:1. is it easy to get places rented out there or is there not much demand, 2. how have rent increases been over the last 10ish years in the area, 3. is this area generally somewhere that’s likely to see some good appreciation4. any serious crime concerns, or anything else I’m not thinking of!

It seems like rules/guideslines are changing by the day concerning ADUs, so would appreciate some real world experience in the matter.  

My concerns with these are like yours - the constraints.  

Greenwater has a few as well which is on the road toward Ashford.I would be a bit concerned about appreciation.

The main issue with many Philly homes is the age these properties were builtThis give you 2 main concerns:1.

If there are concerns or issues with the property condition or capacity as a vacation rental figure it out within the contingency period and accept, compromise or cancel.

I don't have a formula that I use and I'm much less concerned about cash flow than I am equity, although the potential added cash flow in your situation is a really nice bonus. 

I am more concerned about calculating accurate expenses and how operationally intensive it really is.  

I can understand their concern, but I know people have done this.

If you are more concerned with appreciation and less about cash flow, I'd suggest that you look at Hoover, Vestavia, Trussville, etc.