The panoramic sunroof can fail due to faulty wiring and has also been known to leak in both M5 and M6 models. This is due to worn rubber seals, so it's worth giving them a wipe with silicone reviver when you wash the car, to keep them supple for longer.
Ensure car has wheels is a good start
Water can also find its way into the headlamps, and this is just something you'll have to live with on earlier cars. It clears quickly when the lights are turned on.
The M5 Touring has a powered tailgate, which is reliable. Though the switch for the independently opening glass hatch can fail, it is simple to rectify. It's also worth making sure the glass sits snugly in its frame and doesn't rattle.
PHer's view:
"This is the most menacing car I've ever owned. Most don't know what it is, but they tend to move over on the motorway anyway! Only wish I'd got a Touring for the full undercover effect."
Jamie Wills