About a year ago I renovated my home, my basement was finished but had wood paneling, I went over it with drywall — that’s now where my home office is. Around the four-month mark I started feeling off with some symptoms I couldn’t really explain. I finally had a company come out to test for mold. I thought this was just going to be an air quality test but   
from the moment they arrived, it felt like they were just trying to scare me. The tech walked in and immediately said he could “just feel” there was mold down there, before doing any actual testing. He did identify some legitimate-looking issues — problems with the ductwork, vents, and a closet with visible mold. He said he pulled out a recessed light and claimed there was mold running down the back of the drywall, and said the entire ceiling along the ductwork would need to be torn out and replaced, but I never saw this picture or any evidence of that. He mentioned it would all have to be done under negative air pressure containment to prevent spores from spreading or something like that.  
What raised a red flag for me is that this same company conveniently has demo crews, HVAC technicians, and drywall/painters all in-house. That feels like a serious conflict of interest — they’re diagnosing the problem and profiting from every step of the fix. I haven’t seen the estimate yet, but I’m expecting it to be steep.  
Is this something I could manage independently by hiring my own people? And is it standard practice for one company to handle everything end to end like this?

Advice on remediation