On my agenda I plan to meet with my realtor, a couple of proposed property managers, and visit a couple dozen potential listings.

Think about it as dating... you want to be prepared to propose but don't want to take the ring out of your pocket too soon! 

As example, to cover how much of the proposed rental income from departing will be credited against those mortgage payments.

Hire a Architect to provide drawings with a lot of details such as existing house elevations and floor plan, proposed elevations and new floor plan, roof, wood deck, addition, etc.

In your proposed situation, here is what I would do:1) Make sure you have a good/solid/reliable contractor that can actually do the work and has capacity to handle the project.2) Apply for the max HELOC and get it established. 

My understanding is the seller bought with seller financing significantly below market value and has the capital to pay the near due balloon if needed.The seller is the one who proposed SF due to (I assume) capital gains. 

Harris proposed tax credits for builders, cutting red tape, and a $25,000 down payment assistance for first-time buyers.

I filled out a pre-application form with the Columbia Zoning Department so that I can get a meeting with the committee and discuss my proposal.

Which I would love to propose to him and seriously Id love to sign an contract like this (if he had any balls he would).  

I rejected a hare-brained project proposal, and a -$14k charge stayed on my account forever.