The gearboxes don't fail. It's a knowledge issue.
Read through M5Board, a LOT.
Over the last couple of years, members' knowledge of the whole car has improved vastly.
If you're able to move a spanner and do some maintenance yourself, you can do most of it yourself.
You'll need something like the dealer-level DIS or ISTA software in order to troubleshoot and fix most issues though, including the SMG-related ones.
As far as SMG issues go, the most common are clutch related, not 'box related.
Common SMG failures are:
1. Clutch release bearing isn't lubricated from factory properly, causing premature wear and rough gear-change characteristics. This leads to a busted clutch.
2. SMG pump failure. Although not as big of an issue as it's made out to be, it's a R4500 electric motor that fails.
Clutches can last anything from 40 000km to 120 000km. It's all about the previous owner(s) and maintenance history of the car.
Knowing how to drive it, is half the win. Don't putter around in traffic in D1, for example, as this causes the car to pull-off in 2nd gear. Don't hold the car on a hill with the throttle etc. Treat it as what it is.. a hydraulically actuated manual.
The things to keep weary of, which are not show-stoppers providing that you're a DIY type of guy, are things like the throttle actuators. The nylon gears internally, wear out and cause the whole thing to fail due to thermal overload of the electronics.
You can do preventative maintenance, at a cost of $225 for some replacement gears and save yourself R14000 for a single actuator should it need to be replaced due to failure.
The bearing issue is a little subjective.
There's a lot of data for failures in the US.. but less so for Europe and the rest of the world.
Sure, it's a numbers game, but we don't experience the temperature extremes that the Yanks do.
The general consensus seems to be, though, that the S85 and S65 have an issue with tolerances of the rod bearings.. Tolerances are too tight for the factory oil weight and they suffer from insufficient lubrication.
How many E9x M3's have suffered from bearing issues locally?
I know there are a few, but what is the actual number in terms of percentages of vehicles sold? Is it a significant amount?
Again, read M5Board and go from there.
Although if you're NOT a DIY type of person, it'll be something that could cost you a lot of trips to a mechy, and quite soon burn a hole in your wallet.