I can't answer the specific detail on the Dunlite alternator but I can answer your basic questions. Before the days of decent permanent magnets and solid state rectifiers, you had to use a dynamo for dc and if you wanted an alternator for anything you needed to supply its field via slip rings. The exciter was a special dynamo with commutator and brushes and the exciter brushes fed the brushes of the alternator so you had a commutator, 2 slip rings and lots of brushes to maintain.
With the invention of the germanium and later silicon diode, rectification became fairly easy.
The brushless alternator you refer to has an alternator as an exciter but it is built with a stationary field and rotating armature. It is wound 3 phase and a rotating 3 phase bridge rectifier produces dc on the shaft. This is directly coupled to the wound field ( rotating) of the main alternator.
You now have a simple mechanical scheme and you supply a very small current to the exciter field to control the main output of the alternator. The advantage is obvious for a power station, but you may well ask why use it for a wind generator.
With a wind generator you need a second larger rectifier to rectify the output of the main alternator for battery charging. What do you gain?. You gain the removal of a pair of slip rings and brushes and hence a longer period without maintenance.
What do you loose, you have significant losses in the exciter and rotating rectifier which has to have its power extracted from the wind power input.
Brushes and slip rings are nowhere near as troublesome as often believed but the reliability gained was considered enough to offset the cost and lower efficiency.
How does it compare with permanent magnets? Now with the introduction of neodymium magnets it is a costly and heavy way to achieve the same thing but it does have one advantage, you have control of the field strength and you can manage more easily to match the output of the alternator to the wind rotor, although the overall efficiency may be no better from the alternator point of view you gain from the better tracking of prop tsr.
Flux