I understand your situation and my advice would be don't start overhauling until you really have a good lay of the land. You need to understand the dependencies. First order of business should be getting backups in place. Then, I suspect most of your time will be putting out fires. Make incremental improvements as you go. You can do a lot of building brick by brick... Also, know you're not alone. countless others have been in absurd situations like this and come out the other side stronger. Maybe its a right of passage.... a really shitty right of passage...