How to ”Best Practice” a Document Library to management / keep record of several Project Documents

thomasknebel First thing, folders are not “evil!” It is totally fine to use them to organize your documents. What you want to avoid are nested folders - deep folder hierarchy that requires a lot of clicking to find content. That said, if you have one Library per project, you can use column default values in Library Settings to automatically assign consistent metadata to each document when it is uploaded. If the file already has values for the metadata, the column default will not override what is already there - so you need to be careful if you copy files from other projects to use on new projects. Another option is to use a Document Set for each project. A document set is a special type of folder for which you can also set default metadata values. The difference with a document set is that the values you specify for the document set WILL override anything that is already in the document. 

Bottom line is that folder can be your friend for assigning metadata and, you can create “no folder” views that show your documents in a flat structure - the benefits of folders for assigning metadata and a metadata view for exploring and discovering content.