I know for America $12,900 is a bit high. 8-10k would be the right price depending on options, color, condition, etc. Is it a WS6? WS6 cars go for 2k more than base cars.
Manual or automatic??? The 10 bolt rear end is pretty weak, but should hold up fine under stock power in an automatic car. A sensibly driven M6 shouldn't have any issues with the rear(clutch dumps on sticky drag tires tends to break them, even with stock power).
Ask about the clutch on the manual car. With 84k I'm sure its been changed at least once. The factory clutch sucks and wears out quick.
Also make sure the headlight motors don't grind and the door panels might have a crack in them near the window(they all do that).
The biggest thing to look for is accident damage. These cars tend to be driven by 18 year old meat-heads. Its extremely easy to spin these cars into a ditch if you aren't used to the power.
