I have looked into retrofitting disk onto my CoMotion Speedster. I talked with CoMotion and the front is easy, new fork and rim that is disk compatible. For the rear I would have to ship the bike out to them to have the mounts for the disk added to the frame. They recommend doing the front only to have that extra braking force. I currently do not have the drag brake installed since we don’t ride many steep long hills near me, but if I head to the mountains I will reinstall it. Drag brake is needed on long descents to keep the rims from overheating.
For me the cost of a disk up front is a new fork, rim and the disk brake and parts.