I would caution about buying
used vanes made with polished stainless tubing, particularly if there are welds. While the outside may look good, the tubes may be rusted inside and fail under load. I know someone who had this experience with a very popular brand, and it turned out that the tubing was so far gone that it couldn't be repaired with welding. So check carefully the interior
surfaces if you go that way.
Agreed that getting the right match for the boat is a big part of the problem. This is mostly an issue of getting a matching mount, but there is a bit more to it than that. For instance, if the boat has a balanced rudder then the steering
forces will be low so most vanes should do fine; for those of us with unbalanced transom-hung rudders a more powerful vane will be needed. This means one with a pendulum or trim tab that uses water
force to amplify the wind
force to steer the vessel. Buying
a vane with an integral rudder (in addition to pendulum/trim tab) bypasses a lot of potential issues as it is designed to work
independently of the boat's steering. The models with auxiliary rudders that are directly driven by the wind
will not be as powerful, but can be adequate for boats that can be steered easily.
Just about any vane will steer just about any boat in the 30'-40' range in moderate conditions on the wind or wind abeam - it is at the margins that the designs differ (for instance, on a run in light winds). Also, there are big differences in the ease of course setting or deploying/recovering the pendulum. Lots to think of...