I live on the main unit. 

There's no difference in a local flipping a house than flipping the small mixed-use building that is vacant in town into a couple commercial units and some apartments above. 

Our options are to put the money the stock market, buy a multi-unit property, by an established business, payoff the 3 rentals for cash flow, or get into hard money lending.

Single family, 2-4 unit? 

I’ve got 31 units, and I’m sure there’s 4-5 other landlords with 15-50 units in my area.

I am new to the world of real estate investing and want to one day own properties and rent units out.

Quote from @Gregory Wilson:As for the second, there are only about 20 monetary numbers to be entered in Turbo Tax on a single unit rental property 1065 so it always seems to me that the $3500 would be better spent elsewhere.

 Which city in the United States has appreciated the most over the past 10 years?

Make it a gas unit, not wood.That might make it more interesting.

I think it is different if you live there, speak the language, know the market.