SDL Tridion will allow you to edit any Multimedia Component (you can download the file and upload it back again after a change, or you can even open it directly via WebDAV if your editor supports that), you can implement workflow for alerts after a change, and it will keep a version history of your changes. So I would say that already consists of basic DAM functionality.
If you require more, then you will have to choose a DAM vendor for that, and that you can connect via ECL. When you switch over from (normal) Multimedia Components to ECL items, basically the only change you need to make is exchange the Multimedia Components you link to, for the corresponding ECL item. The ECL item will be based on a different Schema, but that is in fact just a Multimedia Schema. Same as inside the CMS, your ECL item will be referenced by its Stub Component, which is a normal Multimedia Component (with an external reference to your ECL item).
So if you already know that you will need DAM functionality, you could consider choosing it upfront and start to work with ECL right from the beginning. But it will be possible to migrate something like that at a later stage too, since all in all the real process of working with a Multimedia Component or an ECL item aren't all that different.