BiggerPockets also has a calculator to analyze deals, and I highly recommend you start this as soon as possible, even if you are not ready to buy.

In calculating the actual construction loan amount will it be the 50k+construction costs?

If the improvements were recognized as taxable property before, you might be able to keep the property taxes and depreciation setup.

 $250,000 capital gain per spouse is not taxable.  

All the BP calculators I've seen are based on LTRs. 

For market pricing, I collect LTR data from BP rent calculator and multiply by 1.5x.

There are several websites available today that will help you to calculate potential rents.

It's worth looking at a blended rate calculator before you make a final decision of the type of loan as it will show the new "blended rate" of your HELOC and first mortgage if you go that route.

I’ve even had structural engineers say that a deck or other structure should have fallen down as well, according to their calculations.

The 1% rule is a simple rule which is easy to calculate and describe.