Here is a link to Jorg's R80GS page, about final drives: http://jhau.maliwi.de/mot/r80gs.html#finaldrive
I agree with his assessment. Unless you want to change the gear ratio or do something custom, getting a replacement final drive is better and easier than rebuilding your current drive.
Opinions vary, and someone may come up with the 'best final drive rebuild company'. But it is not a home repair job for someone unfamiliar with the process and the instruments and tooling may cost as much as a good rebuild or another good used final drive. YMMV. Good luck!
Edit: here is a link from Snowbum's site on rear drive information: http://bmwmotorcycletech.info/rearwhlsplines.htm
p.s. Australian dollars are more than US dollars and shipping is expensive to Australia.