The transmissions seem to be pretty solid. But, there are not a lot of
them out there so you may not get a lot of responses. They are filled
with synthetic oil (use only Volvos') and then sealed and probably best
just left alone. I have not had any problems with my transmission on my
1998 S70 T5M. Since the clutch goes for a long time I doubt anyone
has an 800 or 70 series with a third clutch in it so you probably won't
hear much on life expetency (trust me, other things will break first).

My probelms have been with the clutch. It squeels as it is let out.
Others have reported this problem, too. If this C70 has this problem then
it is easy to hear when first gear is let out. The good news is that it
does not seem to be a problem in terms of the clutch continuing to work
(mine has been squeeling for 30K+ miles). I would think this issue would
have been sorted out by 00 model year.

60K miles is not much on the clutch. It should go twice that before
replacement. Replacement is labor intensive (figure 8 to 10 hours
of labor) so it is not cheap to replace (which is why I'm not replacing
mine for the squeel issue).

Overall, the powertrain on 800/70 series is pretty robust. Bolt on
parts (like AC) are the problem areas. I'll let others go into details
on those (or you can just search the archives are recent posts since "what
should I look for" is a common question).