I count the appliances, roof, paint, ac/heater, water heater, etc and googled average life span of each, estimated cost of each, and then average it out to a per month cost. a rough estimation for a duplex according to my estimation is about $150/mo.

A mortgage could have been taken out 10-15-30 years ago and I know of people who had paid it off in that time span.

Note that you want to study it for a long time period, spanning multiple up and down cycles ... this will give you a good idea of the way the market has behaved over long hold periods ... tough to figure what it will do next year or in any short term timeframe, but much easier and more accurate to predict what it will do on average over long holds (10+ years) ... telling new investors to run the numbers, and then telling them to ignore some of the most important numbers is really bad advise IMO ... yes, run the numbers ... run ALL the numbers (not just some of them and ignore the others) and compute IRR.

Your HVAC cost for that size SFR is $5K (there is often a premium in summer) and that you believe HVAC have on average a 20 year life span (this is the number that I actually use for SFR HVAC - furnaces last a lot longer). 

Macroeconomic trends tend to span more than days or weeks.

If you had to choose between sending out 20,000 pieces of direct mail in one month or 5,000 pieces of direct mail over the span of 4 months.. which option would you go with and why?

Life is pretty short to think about investing 5 years into the future, which is a large percentage of someones life span.  

Found my first two (of many) deals in a span of 12 phone calls within a day from each other.  

I didn't even read your whole post as it was too long for my short attention span, but the first thing I thought was if I finish the basement, I'm just encouraging more people to live or occupy this space which increase wear and tear, and sure you'll get a few bucks more, question then becomes is it worth dealing with the headaches afterward.

This year has been packed with life changes for me, as I recently got married, bought a new home, started an entirely new career as a Software Developer, and got licensed as a Real Estate Agent - all in a span of 12-months!