as good as cantis or not?
Not. I'm sure there are modifications that can be made to improve them (and I'm sure Geoff Apps can tell you far more about that than I can) but stock Sturmey drum brakes aren't good stoppers. They're mediocre in all weathers with zero maintenance though, which is their advantage.
in terms of durability or speed of gear change or what?
I really meant the brakes. But for off-road use, this doesn't seem to be a great compromise - you add a hell of a lot of weight and end up with a narrow gear range, limited tyre choice and mediocre brakes. For an errand bike it's a different matter, but it would make more sense to start with a frame (Post Office surplus?) that's built to take the wheels.
Calmes dans le demi-jour
Que les branches hautes font,
Pénétrons bien notre amour
De ce silence profond.