Without the prepay, the rate will be adjusted and your desire to "run very lean" will be mitigated. 

Pex IS NOT a substitute for insulation/heat tracing in cold climates, in areas where plumbing is exposed to the elements.

On the other hand, taxes are low and inflation is substantial (3-5%), and as rent is adjusted yearly for inflation, neutral or even negative properties should be flowing positive next year or within a couple of years.

Adjustments made for differences or issues with availability of recently sold properties. 

Assuming it will take 4-5 years to finish project.Land Cost 200k8 unit building cost 695k4 unit building cost 350kTotal Project cost 5.415 MillionThese numbers will be adjusted 60 days before each building (exact bids) but are based on builders previous builds of exact same buildings recently.I try to look at investments more with Cash on Cash return versus cap rates.

Some other items, in addition to those pointed out by the OP, that point to continued rent appreciation include increasing minimum wage, increased interest rates making it harder for first time home purchasers, near best climate. 

If the PS + ABnB is netting higher profits but at lower percentage vs your straight B+H strategy, you might want to adjust and run the numbers on a B+H strategy in PS. 

If you're originating the loan yourself try out each servicer and adjust in the future if your not happy.

In the name of 'asset protection', you will forever be subject to crappy commercial financing with short terms, balloons and adjustable rates.   

Maybe look for off market deals or adjust your expectations.