It's impossible to know for sure without actually examining the plane. It doesn't sound like a deal breaker at all, especially since you got a chance to look inside the gearbox. Don't take this wrong, but do you know what you're looking for? Do you know what a "smoking rivet" is, or what aluminium oxide "paste" looks like, and what can cause it? If not, you need somebody else to look at the gearbox area. The flaw could indicate a hard landing at some time in the past (virtually guaranteed on 30 year old airplanes), but a good inspection will either dispell any fears, or identify a bonafide problem needing correction before purchase.
My best advice to you is to use the forums search engine and read everything you can find on buying a plane before any money changes hands. Lot's of folks here have given lots of genuinely great advice in the past. Don't forget to change the date range to 1 or 2 years in the search.
