Hey Peter,
My Canonet is one of my favourite cameras to just haul around. I have tried both mercury and alkaline batteries in mine, and both will work perfectly well. If you can only get alkaline or wein-cells, I would suggest alkaline...They are a lot less expensive and last a good deal longer. Regardless, if you use mercury, alkaline or wein-cell, you will have to calibrate against a good light meter. Even my recently CLA'd Canonet with a mercury battery is 1/3 stop off. The important thing is that it is linear.

Hope that this helps,
Kent