No need to take on extra headaches.

How much extra/emergency money will the bank want me to have available to carry these two mortgages?

This is true if the duration is 5, 10, 20, 30, 40, 50, 60 years; it does not matter what number of years you want to start the basis at.I do not know your current living situation but knowing that I am pro investing local this is what I would do: 1) Look for a detached duplex local (in OC) that has sweat equity but can still qualifies for an FHA loan.   2) Purchase the detached duplex using the FHA loan (at 5% of FHA down your $50K will go a long ways) and house hack the duplex living in the more thrashed unit 3) Rent the "bettter" unit while referring to yourself as the Property Manager (not the owner) 4) while you live in the more thrashed unit I would work on earning the sweat equity (rehab the place) 5) when done rehabbing the more thrashed place I would consider if I want to keep the tenant of the other unit. 

I'm working with an investor/partner who has pledged ~$300k of his own money to help me get started while I run the operation earning sweat equity.

FHA guidelines might allow for your situation, if your a non commissioned W2 employee, self employment or commission in income requires an earnings period to average over.

You need earned income, e.g., income you'd put on Schedule C. 

We have also seen them set up 401k's for client's who did not have and did not plan on having qualifying ("earned") income. 

However I don't think that earning a 1.8% CoC is anything significant...better off investing it in bonds/stocks?

1700, put extra toward down payment at end of term.

Paying extra for the right people, big picture, is actually the lowest-cost solution.