In reality, I’ll be able to rent out one of the two units, and I will live in the other unit with a few friends, so I’ll actually be living for free & positively cash flowing, with a good cushion for potential issues/disasters, but I know that’s not how it’ll appear on paper unfortunately. 

The fact that you can measure the thickness of the financial cushion you've built between you and poverty also helps lessen the sting of the years that you spend clawing as hard as you could to get somewhere.I think a lot follows from being clear on those two points starting out.

I think the buffer has to be enough cushion to allow for your biggest possible expense.

If they want to stay they and pay me a fair price that's fine, if not it gives them a cushion to leave.

I would wait until you have at least 20 % down payment before purchasing another property, unless your main day job can help cushion the expense cost (vacancy, repairs, taxes, insurance etc).

As was mentioned when one life emergency happens low income tenants do not have the cash cushion to handle it.

Those are the people that I think will either validate or make you re-evaluate your calculations.Try not to make pleasing the seller an issue with financing, If you go trying to support a 10 year mortgage when you don't have the financial cushion set aside or cash flow generated for repairs, CAPX, evictions etc. 

It’s wear and tear on your couch cushions.

Some common mistakes are trying to go cheap, not vetting out contractors, paying contractors too much up front, not having contractors sign a contract when you accept their bid, not having enough of a cushion for unexpected repairs (there will always be something)

Just build the average utility cost plus a cushion into your numbers.