If I was selling a duplex that I expected to sell in 2-3 months max I would leave one side vacant in hopes of selling to an owner occupied buyer, or at least to make my buyer pool as large as possible, hoping to get more out the sale than selling strictly to an investor, who is the only type of person who would buy a duplex where both units are occupied. 

My point is that if skyscrapers is your niche of choice, don't expect to close many deals.

If your budget is $830K, then you can expect to build 1,000 sq ft units (small 2-bedroom units).

And since you just did this "recently", I don't know your level of experience or expectation well enough to say whether you should/should not buy more property.I recommend first you give us more information.Also, be sure to clearly define your goals. 

We were expecting to use the security deposit to help cover some of the costs but the property manager never collected one even though the lease required it.

To me that is to be expected....................................just read through listings and prepared summaries and you soon realize expense numbers are incomplete, out of date, or just plain wrong, NOI is always wrong, so why would you expect a correct cap rate?

On a fairly straight forward RE deal there probably shouldn't be a ton that can go wrong to begin with, but if anything does, why would you and your father fall out over lower than expected returns or losing a couple grand?

I would probably expect them to at least have some type of debit card..... if not then that would put Venmo out as an option - but, it would be easy for them to do right on their smartphones that you mentioned most of them have.

Best decision ever, it has worked out better than expected. 

If they're MLS listed, expect them to sell the next day for 10% over asking price.