I think you are correct if it would be a larg(ish) turbine, but at first I want to start small. It doesn't even have to last long, it's just experimental.
Remember that on a VAWT the tip speed is proportional to the wind speed. So smaller turbines have higher radial forces on them from their spin, in inverse proportion to their radius. (Like a car at a given speed taking a wide or a sharp curve.) A smaller turbine has to be stronger than a larger one or it flys apart.
That's mainly an issue for the prototype's structure. But there is an issue for the bearings:
Faster spinning means more viscous heating of any lubricant. It also means more wear (though wear is reduced by the lowered load forces).
Wearout shouldn't be an issue for a short-term prototype, but you'll have to deal with it for a more permanent instalation.
(I'd be inclined to go with automotove bearing systems for a working mill. They've been designed for long life despite heavy loads, far heavier transient loads, and extreme conditions of weather and dirt.)