You will be exempt from 1/4th of the gain up to the limits of the sec 121 exclusion assuming that all units are identical and your accountant set up the depreciation schedules with those equal allocations.2.

As far as your question on continuing to purchase more properties, if you're buying good properties that make sense from an investment perspective, your rent should net out most of your mortgage payment and help keep your DTI under control.

My experience is fairly limited to one location, but based on that 15 year is the max. 

The property is located in Los Angeles where it's subject to rent control.

We brought a SFR with a newly installed roof but there were leaking from first heavy rain. The roof company came to fix it for many times but still leaking and then we were told that is the sweating of duct piping. We...

Be sure to polish up your 'negotiating skills' so you stay in control during these conversations!

At this point, I prefer to have deals closer to home (Chicago area) though I realize that may be limiting.

Call the HOA to see if they have reached their maximum rental limit before you buy. 

I have told 2 guys in the past who had nice deals , lets do it , BUT  51% control goes to me , I call the shots .  

you can have 10 mortgages with fannie, the restrictions get a bit tighter on 5-10 but if you have strong underwriting should be no problem. many banks will limit how many they give you, but there are no shortage of banks.