Black death is when the injector seal (to the head) passes. The black gunge escaping is a bitch to remove if left to develop and bake.
It isn't as scary as it sounds and is fairly common.
You should smell it and hear a put put of gas escaping at tick over, but not always heard as can be subtle or diguised.
A serious test drive looking for vibes is of value.
Some appear as temp rises, most will show under load at around 1.8k revs+
Vibes can come from many places and be an expensive problem to resolve as it's often trial and error.
Also look for gear box slip when cruising at a constant speed. I've experienced this at 58 / 60 mph and seen the revs rise 200 and fall back. There is perhaps a little vibe to be felt at the same time as one of the few TC or gearbox clutches slips and catches again. It's reasonable to reckon that will get worse.
The gearbox fluid and filter should have regular changes, although the beleived 40k mile interval isn't set in stone.
Pros and cons though as the manual box will have a dual mass flywheel that 'can' be problematic.