The ideal generator will be a large four-pole one that weighs at least 40 pounds. A few semi type trucks and old city buses used such generators. Even though these generators will be 20-30 years old, they are generally quite serviceable after replacing or repacking the bearings and replacing worn brushes. Sometimes the commutator will need to be turned down on a lathe, a job that isn't expensive. Here are some examples of 12-Volt generator specifications obtained from truck repair manuals in the local library.
Some generators even have their rated output at a specific rpm on the generator tag.
The most desirable generators are the ones with the lowest rated rpm since it will lower the wind speed at which the generator starts to put out usable power. For high winds you need a governor. A governor can change the pitch of the blades at a certain speed or it can act as a drag brake, much like the simpler Wincharger-type air brake governors. Another type of overspeed protection is to have the tail fold parallel with the prop in high winds. Folding the tail manually is also a very good way to shut down the wind plant during periods of high winds.