I installed for a couple places a few years back, one of them a family member at his daycare center. It was only 4 cams but damn it was a lot of work for just me running those lines. It's a an eyesurv system from Nelly's, back when they had dahua stuff. The one I changed out was a parkinglot camera where the dome got condensation in it enough to the point that it got to the electronics and killed it. It may dry out and come back though. I put the same type back up, only new. Hopefully it lasts. I would have LOVED to had time to get a few of these startlight turrets from Andy, but I had to get something back in place. This is the video I did of the install back then- I'm not a pro by any stretch but it has held up well since 2013 except for that one pesky camera.
As far as doing installs for even a side business, I wouldn't advise it. I was very into starting that at the time and I'm so glad I didn't do it. The main reason I dropped the idea is because of servicing. All of this gear gets pissy in a hundred different ways over time, including the customer creating their own problems- tweaking settings, changing routers, you name it. I had no interest in spending my off time chasing all that shit down and the more you install, the more you're gonna chase shit down. Not for me. Plus without being licensed and insured and not really knowing construction, I was wary of popping holes in peoples houses and all. I know my limitations- I kept it as fun for just me. Think it through before you commit.