The single action only feature was a factory option for the K-38 and I own one. The single action only models were supposed to give a better trigger for Bullseye shooting. DA parts can be used to replace the SAO system and the gun will then be a regular K-38. The factory built guns in this configuration and SAO kits could be purchased from the factory for conversions back in the day.
It's hard for me to give you a good answer as living in California puts me somewhat out of touch with what guns cost in America. There was some discussion about this on the S&W forums recently. If the gun is nice and had the box I personally would leave it alone. If it is a shooter grade gun then I personally would probably still leave it alone but I think I'm probably in the minority as it seems not many people are familiar with these pistols and see the lack of a double action feature as a disadvantage. From this collectors standpoint, there were so many factory kits sold that you are not going to know if yours a factory version (unless the original box is so marked) without getting the revolver "lettered" by Roy Jinks. If you change your pistol to DA you can always save the parts and change your converted revolver back to SAO later if you desire.