I have a 1990 300E that's done 120,000km. It's sweet in every way
except the transmission.

Changing gears seems slow and the engine often revs when changing up.
I'm told this is probably due to worn friction discs. I bought the car
six months ago, when the problem already existed, so it's difficult to
know how long the problem has been there.

More frustratingly, when changing into reverse, it can take quite a
while for reverse to actually engage. If parked on a slope, the car can
roll forward several feet before reverse takes hold. It's a bit awkward
and I often need to use the handbreak and rev the engine in an effort
to get into reverse.

Will I be best just to replace the transmission, or can it perhaps be fixed?

Thanks
Matt