As a Goldwing guy, let me put my two cents in....
The alternator on those bikes is buried in the engine, on the flywheel I believe.
You have to pull the motor from the bike and open it up to get to it.
And....it's a 34 year old bike. Will you even be able to get an alternator for it?
I don't know.
What I do know is....
The problem was likely caused by adding too many lights and gadgets
to the bike. The alternators were not strong enough to support a lot
of extras. So the alternator problem is not uncommon.
Many people have converted those bikes to run with an external car type
alternator. It's called "The Poor Boy Conversion"
There are several Goldwing Only sites that talk about this.
I would start with the "Naked Goldwings" site and see what they
have to say about it.
The good news is, if you fix it and learn to ride it, that bike will
likely run forever. He doesn't say how many miles are on it, but
it really doesn't matter if it has 30,000 or 130,000 miles.
Those bike are known for going hundreds
of thousands of miles with no problems other than the alternator.
$800 is not a bad price if you can do the work, but I'd try to get
him to go even lower.