But if the appraisal comes in lower (or higher) then you can adjust your payoff. 

You'll need to make adjustments as the utilities cost changes.

I know they adjusted voucher limits higher again this year and we'll be pushing for increases on ours when they come up for renewal. 

Lets shoot for $90k down, $150k gross per year, adjust for your COC calcs, once you throw in expenses, Debt/Interest.  

Use the public records for your offer, and negotiate hard, I would add in the special provisions that offer price is based on $x/sq.ft if the house is smaller than the listed sq.ft the price will be adjusted based on the sq.ft. 

Regarding running model / proforma - simplest to create something in excel that you can adjust the numbers on.

Auto or manual on /off?

That doesn't necessarily translate in high real estate investment returns but you can make some money investing in real estate there (like it seems you do) and so it makes perfect sense that you do that since you live there.Those like me who don't live in Panama are likely purely looking at figures and comparing risk-adjusted returns with other countries where they can invest.

I thought this calculator auto-calculated the monthly payment I see now that it didn't and I adjusted it to be around 3.7k and now it is making around $600 a month which seems more reasonable.

I’m going through my private adjuster so is making the claim go slower than wanted.