The price sounds OK for Adelaide. I don't think it would be all that much less in Sydney.
Maybe not whining diff, maybe CV or wheel bearing. Do what previous posters have advised and have someone have a look at it so you will know exactly what you are up for.
If you have looked at Bruno's site you will have seen the things that can go wrong with these cars, but don't let that put you off. Ask the owner what he has fixed/replaced over the years.
There are many high kilometre E34 owners (or at least their cars) on this site with 300k, 400k and more kilometres. If you intend to buy just go in with your eyes wide open.
That's the beauty of this site - benefit from the shared knowledge of the people who have been there done that.
My '89 525i has 280,000km on it and I expect to get it up to 400,000.
Phil