Buy it or steer well clear?

Nope, but I know people who were. I seem to remember one of them doing the same thing to a small block Chevy engine too.

Brushes wore out at 80K on my 1984 Celica XT. They cost about 50p each and just undoing 3 bolts and a bit of leverage got the rear cover off. And then the new brushes could be soldered in. Had to drill a small hole in them to fit the nail that slid though matching holes on brush holder to hold them clear of the slip rings as the rear cover was re-fitted. The holes were away from the rubbing end of the brush.

1989 Nissan 200SX go 120K miles. I know this as I have 2 in the shed awaiting brushes. Utter pig of a job, needs a 200W soldering iron to heat the bearing retainer and release the rear cover. My biggest iron is 80W. They say it mustn't have direct heat like a hot air gun or blow torch. Then the brush lift wire holds the brushes by the contact face and only engages with holes each side of the holder. This means that once the wire is out of the far hole it will be pushed on to the slip ring, possibly leaving a score mark.

The highest mileage alternator I know of is on my brother's BMW E28 520. Well over 250,000 miles.

I've no explanation why some brushes apparently wear out while others don't - unless it's dust etc being drawn through the alternator. And the position on the engine may make a difference to this.

[..]

Harmonic vibration causing brush bounce. It's sometimes a big problem in industrial machines.

Chris

Quality of the components used , engineering tolerances , the way you drive the vehicle (miss daisy as apposed to Jensen button) damping of vibrations .

One in my olde Audi A6 hit 245 thousand odd...

Which is odd. Either they wear out or they don't. I was surprised by the comment since I've never had an alternator fail due to worn out brushes.