It sounds like your question is what is the conventional guideline on using these rents to qualify ?

I'd recommend looking at costs. your build cost is going to be well over other homes because of the fixed costs like slab, Hvac, roof, etc. check the market drivers and weight the benefits of short term versus long term. long term is no furniture and help pays mortgage but at this size it doesn't really cover everything. we didn't find a lot of economies of scale in single family detached. can you build more than one unit?

Like you said we have been saving upwards of 60% of our W2 income and business cashflow in order to do this.  

Join one and start volunteering - it’s the best way to network and meet like minded people.

It sounds like you are just having trouble pulling the trigger. 

We're looking to gather info about likely cost, pitfalls, time required, considerations when obtaining financing, etc. 

Many people don't buy because they are concerned about this and that.

Lastly, if all this process seems like just too much and not for you, I suggest talking to @Chris Clothier that's who I'd personally go to if I wanted to make some market purchases off someone's word of it.

And technology aside, this business hasn't changed much in the last 20 years.

Has anyone done this and have had success?