Well you do have a good grasp on what they are about. There are many different DAC's out there in various price catagories.
Your CD/DVD player must have a digital out to take advantage of an external DAC. A transport is basically a CD player without the internal DAC. All digital devices had some form of a DAC, hence the name digital to analog converter, DAC.
To address your concern about SACD, basically no it can not be done today with out lots of money. I believe there maybe one or two which do have it but a great expense. The issue is the difference with the "signal" produces and the "copyrighted method" in which it is derived. (Sony & Phillips)
I not sure how easy it is to use a DAC with a DVD player but it is possible.
I hope this helps a bit. I know it is simplistic.