I have owned a Model 50 for many years. Still like it. Cookshack put a new thermostat in it a few months ago when I was picking up my new FE100, and it works as good as new.
I have never tried to cook fish. But with your stated goal of precise low temperatures, you may be disappointed. You will have at least 25 degree swings, and possibly as much as 50 degree swings in the temperature as the thermostat cycles. (But if you check most residential kitchen ovens, you have the same kind of swing).
I am also not sure that you will generate enough heat to cause the wood chunks to smolder and smoke at a 100 degree setting. I have never done it, but I think cold smoking is done with a tray of ice put into the cooker between the heat and the food, then the cooker is turned off after sufficient smoke is generated.
Model 50 makes a great holding oven for the home cook if you ever put on a big party too….
As to the $600 price, I think I gave about $800 for mine brand new back in the last century.