In addition to the mortgage, property taxes, and insurance, the projected calculations are taking the following expenses into consideration (5% rate for vacancy, 5% for repairs/maintenance, 5% for CapEx, and 10% for property management; otherwise the income figures would be higher. 

I have a duplex I own free and clear, and just need some additional money to finish rehab.

How do I handle the rehab with renters in the house or is it advisable to let their lease end and have them vacate before I take on the rehab (since the additional bedroom and bath would be for a new tenant anyway? 

The additional capital helps too :)

There are often hidden costs--such as: engineer certification of the structure (depending on what the city requires) smoke damage to portions of the unit(s) that are "not affected" additional insurance during the rebuild, sealing the framing that has smoke, etc. 

Who would have guessed Wells Fargo takes 2 additional business days to wire out after charging $30?

In addition, learn everything you can:1) Read books2) Listen to podcasts3) Go to real estate meetups4) Find ways to add value to a potential mentor.

As your assets increase you may want to get a personal umbrella policy (PUP) to cover any additional liability.

On the inside, think about every doctor's office you've ever been in: the major additional system comcern is that Plumbing is everywhere.

It also creates some additional costs (setup, maintenance, tax reporting) and will limit your financing availability (more expensive commercial loans).Depending of your threat level and the amount of asset that you are protecting it may be worth the cost.