Would you factor in a rent cushion, if so how much?

Even if the  weekly/monthly visit is just a drive by / walk around and not a trip inside, it still allows some things to be spotted.   

We factor in about 2.5% of purchase price for yearly taxes, 5% vacancy (ours is closer to 2%) 15% of rents for repairs and capex combined, we self manage so no PM, and use 2% for rent growth (ours is about 4%) and 2% for property value growth (ours has averaged about 3.75% in our area over 30 years). 

My opinion is that this is really a personal choice and a factor of you and your wife's outlook on risk vs. opportunity. 

The plan is to monitor with Arduino-based moisture probes and RPi 'dispatcher', which drives a pump via sonoff switches.

sounds like you have a great opportunity. i recommend the contact for deed route. as you mentioned you can get a lower down and more flexible terms. if you can get a 10-12 year ballon payment you should have no problem refi'ing out prior to that with a conventional loan and pay off owner contract. i would try to get the per unit down. you could try to show the total amount you would be paying when you add interest into the equation and see if he will come down on asking price, at least to market if not below. as well as play up the continued cash for with zero management headaches. ultimately if it cashflows well and you have good fixed debt for 10 plus years my experiences has been that price per unit is less of a factor. cash flow is king.

Perhaps go the direct mail/drive for dollars route and see what turns up.2.

I’m curious as to what other landlords position is on mailboxes and mailbox posts when they are damaged from cars hitting them, very possibly not the tenants fault as it is often a car driving by that hits it or backs into it.

@Solomon SmithStart in Nashville and an hour drive from Nashville.What you are wanting to do is 100% possible in this marketAny market you go to will have it's own unique challenges.

It was tenant occupied, drive by requested, showing after offer and proof of funds.