As for negative interest rates, I could see the possibility of the FED charging banks a negative interest rate on an incremental basis.

I can get pretty decent windows at Home Depot for way less than what some of the Window companies want to charge.

Or is this a scenario where I might legitimately charge some small extra amount for the basement (such as for storage) - or otherwise just keep it locked off separately from the rest of the house?

They may charge you a small fee but it's worth it. 

What's a fair return we should pay him or what's fair for him to charge?

Do You Offer a Discount for Vendor Services or Do You Up-Charge?

@Drew AdamOnce you close, I would meet/talk to the tenants and let them know that you will need a new application (I would not charge them) and vet them like any new tenant.  

Most companies I've looked charge a % of the monthly (or weekly, etc.) rent.

I get info from REIN but the prices they charge compared to what you can get at REAG - hands down, I choose the latter.  

I get 2.5 points charged with each one of the two loans, I have no leverage to negotiate the 30Y mortgage rate, no closing credits and if I want to refinance with another lender in the market I'm charged a hefty fee that will make the other loan unattractive anyways.