I know some will disagree, but once you're 'inside' a piece of equipment like that, I would be inclined to think that you're then in a typical industrial atmosphere.
Given that assumption, I would bet that those brushes, and the gear, are pretty typical industrial components.
Perhaps in the future you could check industrial suppliers for the brushes and for the gears.
Also, 30% mechanical wear off of a motor brush doesn't equate to 30% reduction in service life.
Because you need (about) 30% of the length of the brush to hold it in proper position, therefore you can only use the remaining 70% of the length (from when new), so I think those brushes are approaching 50% of life expectancy.
I don't claim to be an expert, but you mentioned that the brushes were replaced once, so perhaps you could check the brushes at the end of the '10 sailing season?