I don't really want to invest in a nice camera.  

. - camera the sewer lines - get an electrician in there to check the wiring - appliance audit - how old, how much to replace, and any repairs done? 

Every once in a while an issue happens where I wish I had a security camera (still need to install some), but normally I just deal with anything when I'm home.  

My plumber found out by inserting a camera that my city sewer pipe has roots in it which is causing my house sewer to back up.

Get cameras and cooperate with your good tenants.

For people who do own apartment buildings of 6+ units now, do you contract ADT or some other online security system with cameras? 

A sewer specialist will inspect the system for you for about $200 and stick a camera in the pipes (drain tile).Drain tile can get damaged for example by roots (Willow trees are really bad and will go a loog way to reach the pipes), but sometimes its just age.The way you deal with the issue depends much on your strategy.

The sink and tub clogged up again on that day, so I called the city, they came out that day and ran a camera and found that it was clogged on their side, the next 4 days they dug up the side walk, my yard and the street, replaced the pipe on their side and fixed everything.

Installing security cameras? 

We've had a robbery claim as well.About 25 years ago, someone broke in and got off with a lot of our belongings (electronics, cameras, jewelry etc).