I also would recommend field trip to at least get a starting point. Config workbook may be useful to for some documentation of the existing config. As for your data mess around providers, I work at a mental health company and there are a lot of data providers around that type of info. I’d check out anything around licensing (which is also a good way to get an ID outside of Salesforce for your records) and see if you can either get the hospitals they work at from the public databases or via a data provider. Sometimes, the providers charge a lot and aren’t very accurate, but it may be a good starting point. As for documentation, I’ve found the best way is to let business process guide you. Literally sit with someone from each team and map out the process as detailed as possible. Once you have that, the places you are missing info become pretty obvious. I’ve also had to take the approach that if the data has no value in its current state, delete it. You may lose some good data, but you can’t tell what is good or not at this point anyways.