If you want to automate your work with documents, the first thing you should do is not to write your own Document Management System from scratch, but to look around and try to use any of the existing platforms (Alfresco, FossLook, SharePoint).
It is important to understand that the standard solutions may not work for you at first. You will need to customize the processes, perhaps even to consult with the developers of the Document Management System.
A good DMS system should allow you to work and design your own document types, as well as provide convenient and reliable search engine.
Many people believe that the document in DMS should be represented as a file. In my opinion, this is a big limitation. Let's say that you are preparing a presentation. It can include several different .pptx files, right? So, you already need the system that can work with multiple files. Or, another example, in your workflow, a typical document may have additional agreements that need to be attached to it.
So, ideally, you will need a system that allows you to manage a document card with attachments. Be prepared that, most likely, you will have to pay not only the price of the DMS, but also the additional price for the integration services to achieve best results.