Also, I believe $900 is a “safe” rental price for the area D/FW unless in a bad neighborhood, a good area could command $100+ more each door.  

I will be able to knock on the door for the first time, later today!! 

You could rent out your current house if you buy another house to hack - and if your next house is a duplex (or more) that gives you 2+ doors right off the bat.

I was told here and there that renting out a SFH that has keyed entry bedroom door knobs (standard single cylinder and turn-button on the other side) is a fire hazard and against some kind of code or, if not, makes you liable for fire hazard dangers. 

We primarily do showers entirely fiberglass with nice doors.

My ultimate goal is to fund enough doors to either live off my R.E. and quit my job, or be able to do both and enjoy having the money necessary to walk away from a "real job" one day and do whatever I want.

I’m looking to by tax liens and multi family houses now, and rent out per door to certain populations.

Technically, under this scenario, the vent does not appear to constitute a "structural" change, it may also not constitute a change to the exterior (it depends on what the HOA guidelines say), most of the time an exterior change has to do with adding/deleting doors, windows, and changing the  facade and color.   

Unit is in good overall condition, one interior door is damage, and a few small holes - $32K MLS-2180280482123 Dawnlight Ave - 2 BR Flat - Rented $615/month - Great condition, home has been converted to baseboard heat.

You also have to look at the whole picture...how much are taxes in that state, are utilities insane, what can my average rents be per door, what is crime in the area like, etc.