I live in Finland, and as such the weather right now is pretty cold. Since I rely on my bike for shopping in a rural area, the frequent and complete loss of drive I now suffer at below-freezing temperatures is intolerable. The local bike shop has looked at it twice now, and utterly failed to fix the problem; I don't think they've tried very hard. When kept in a warm place overnight, it works fine, but as little as a quarter of an hour outside is often enough to make it fail again.
My own research so far indicates that I have a roller-clutch freehub (ie. it doesn't tick when coasting), which is in general more prone to failure than the ratchet type. Most of the mechanical fittings on the bike appear to be from basic Shimano ranges; it was an ex-rental. The model number on the hub body is FH-RM30, and the gearing seems to be an Alivio 8-speed at the rear, and a Nexave 3-speed at the front.
I'm keen to replace the freehub with a ratchet unit for reliability and longevity. However, I'm reading suggestions that the two tend to use incompatible interfaces to neighbouring components.
My question is: how much do I need to replace?