This is a nice review for this class of cameras:
If you want to use them seriously, take into account the need for a cleaning of viewfinder, and a realignment, maybe, of the rangefinder. That should be around another €40,00. Your mileage may vary.
A battery adapter is "needed" for some of those cameras if you intend to use them at their best possibilities. Using a modern battery without an adapter can give you some slight metering mistake, which is not a problem if you use negative film anyway. Understanding how a light meter meters is more important in getting good results. Use of a separate, external incident light meter is IMO always adviceable and gives best results when time is on your side.
As far as battery adapters are concerned, I suggest reading this, and maybe ordering an adapter from the maker:
Regarding your purchase, I would focus more on the general state of the camera, and its price, than on a specific model.
I would like to add that the quick loading feature in the Canonet is a nice thing and I don't understand why this did not take more ground. When I change a film I have to sit somewhere and put the camera on my thighs as I need both hands to properly insert the lead in the receiving sprocket, the Canonet is the only camera I have allowing me to change film while standing. That's fast and secure.