First, don't panic, it really isn't too bad. I have done jobs like this where the entire side of a two story home was starting to collapse, and the headers above the windows were the consistency of potting soil. If you are a competent DIYer, it's a fairly simple process of removing and replacing individual studs and plates, one piece at a time. I would take a pump sprayer and soak everything with bleach, at this point, then decide how you are going to proceed. Genuine "mold remediation" contactors can lead you down one hell of a wallet draining rabbit hole, so decide how far you want to go with that before you get talked into a huge expense there. If you can, find a local carpenter/handyman who can tackle this, by the hour, and see what they think of doing the work, if you need assistance. Good luck.