When we upgraded the DC at our local library, we were faced with a similar issue: Migrate the existing domain structure or start from scratch. In the end, I chose to just start from scratch and rebuild everything. This was for several reasons. 1) I was not there to know all of the details of how the original domain was created, and didn't feel 100% comfortable with how it was set up anyways. 2) Migrating the DC can sometimes not work quite right and I didn't want to be stuck "half way" and not know what to do from there.
In the end, starting from scratch also had another great benefit in that we could also go around and redo some things on the individual workstations and cleaned a lot of things up and fixed other issues that might be happening on individual computers. Basically it was a ground-up fix of the whole network and starting from fresh. Granted, we had a small number of end user devices like you are talking about here, this wouldn't be near as fun if you had a network with more than like 20 computers.