One thing he used to do was hand-deliver the application to them so he could see how they kept their house. 

The limit on scaling is your deal flow (mine never was that good) and time it takes to perform the value add (I got fairly quick for example, my last rehab did all new plumbing, electrical, and flooring.  

Not applicable unless you plan on living in one unit.

Line of Credits (like HELOCS) allows access to a set amount of money up to a pre-determined limit and it is revolving.

Here in UT we have strict occupancy limits in college towns of 3 unrelated people per unit.

Mortgage lenders will qualify you for a higher purchase amount for multis than they would on a single, because they will count the rent you will be making on the other units as additional qualifying income for your loan application.

You also have to factor in time limitations.

One of the biggest challenges today is the reluctance of homeowners to sell, which limits the available inventory.

The condo will limit the rental options to age restrictions. 

There are a very limited and reliable few of lenders that do true DSCR 2nds.