My problem is being able to determine what I will be able to rent each side of the duplex for.  

Then how much more you do should be determined by what the cost to upgrade things would be versus how much rent you can get if you do them.Separate entrance sounds really expensive depending on your homes' layout.Egress window might cost you too.As for a kitchen, its a small one bedroom in a basement so it isn't like you're going to be getting families renting the place. 

Here are some of my thoughts- I suspect you have not run numbers to determine how it will perform as an MTR.  

The final site for the first Gigafactory will be determined in the next few months, once we have full visibility and agreement on the relevant incentives and processes for enabling the Gigafactory to be fully operational to meet the timing for Model 3."

I am definitely not confident in owner's abilities to determine proper pricing.

Does anyone have any good advice or resources on how to determine an expected vacancy rate for short or medium-term rentals in any given area?

The lower of the current rents and market rents are used to determine DSCR, but some lenders will go up to something like 120% of market. 3.DSCR is calculated with principal, interest, taxes, insurance and HOA (if applicable) vs rents.

My guidance is always to first determine your exit strategy and then reverse engineer into the resources you need, and the platform that provides them, in order to achieve your goal. 

People think history determines the future or create an artificial two way on how the future will play out--not sure why or what or where that comes into play.

The lender wants to run your credit score to determine whether you're Dilcia Morales or not.