So really to have a higher chance at success you should have both money to put in the deal to lower your leverage AND money on the sideline to cover those potential cap ex projects. 

It's kind of like the ex-smoker, in the company of smokers. 

Ex:  Lets say market rent is $1,200 and they are paying 900. 

I guess it's just too many factors to consider and it easy to get overwhelmed and cannot get accurate info and estimation when I conduct analysis (ex. property tax rate, insurance, maintenance fee, rent, repair, closing cost, etc.) 

Because he doubles the price (ex. buys for $40K and sells for $80K to end buyer), he gets a positive cash flow for the next 5 years, while repaying a loan from investor which he uses to purchase the property for $40K.

Thank you. mortgage probably but not PITI, cap ex, management fee etc let alone any cash flow..:( due to high interest unless you put more down

Double check any cap ex you might have coming in the next year or more there so you can plan to set that aside (boilers, roofs, etc).

I would say the cash flow would go up every year and that would usually be the case but my son will be moving into one unit probably for just the labor to rehab the last unit (collected rent will go down for a year).After that rehab is complete, at market rent I would have about $8k/month cash flow with underwriting that most would consider conservative (for example $1200/month allocated to maintenance/cap ex). 

EX: Income $700k (w-2, rental income and dividends and interest) - rental expenses/depreciation $117k = taxable income $583k.

EX: I ran a few real estate ads targeting real estate agents on FB. as a test since I never did an ads before.