The historic return on the SP500 stock index is 9.8% (Investopedia) which will give you a perspective on returns. 

My recommendation is to look at historical averages, increase the rent by a certain dollar amount, and then include water with the rent.

The tenant is nice and respectful from what my realtor says and has historically paid on time.The house is being sold As Is - is that bad or not necessarily bad but I should look for some things, or not a problem at all?  

-This lot was under the minimum size needed to build technically speaking, but still it was a historic "lot of record" it was fully buildable as I verified in person and in writing with the city.

Historical occupancy for the last two (2) years and year-to-date. 7.

Historically, prices flatten during recessions and may see small dips, but if you're trying to hold out and time the market, you'll just end up missing the boat while others continue to build their portfolios.

Further, it has a location on a corner lot overlooking the river and a park (not directly on the river, but right across the street) and is in a historical district.

Because risk doesn’t easily lend itself to quantitative measures, it’s easy to ignore.The fact that an investment has historically returned an amount equal to 12% of the asking price by itself tells me little.

I would say that you need to determine (in no particular order) regulations on rehabbing, sewer/tap condition, asbestos, mold, whether you're in a historical location or not, landlord/tenant laws, gas line condition, etc... all those things can be detrimental to your expense line.Hope this helps a bit. 

That is the historical average for the Denver market over the last 40-years.