Would I be correct in assuming I could knock the 50% down to say, 20%, to net a positive cash flow of $600/month?

You could probably knock some off at inspection too and even more so if you use the selling agent since they won't split commission.

We plan to replace with tankless water heaters and in-unit high-velocity small duct central heat and air (using electric baseboard for the garden unit heat), and also remove all the radiators from the building.With the furnace and water heater removed, we would like to knock down the walls dividing the current furnace room, and make one larger rear, which is the bulk of the remodeling work to be done. don't not currently have drawings or architectural plans.

Ah, the hard knocks of Landloarding. 

No one from FHA will come and knock on your door to verify you're there a certain amount of time so just be honest with the application and it things should be fine.

I'm eyeing a multi-family in my area so I looked up the owner and I'm taking my husband to go knock on his door and introduce ourselves this weekend.

Are you calling, knocking, sending mailers?

There were some great deals with those who had very deeps pockets because it was not just a rehab, you also in some areas were required to knock down, then rebuild fresh or lift and then rehab.

I did some snooping and found the buyer's Facebook page and found out that she was in fact single (as in not married) and had been knocked up by some guy that she had been dating for 2 months.

I knock down wasp nests with a stick.