Pre purchase checks and before you hand over your money - link in my signature may assist.
Warranty is a personal choice and difficult to advise. Zeds have proved generally reliable (with some 'roof motor and rear spring exceptions) and what you pay for a warranty may well more than pay for any problems you experience.
If you decide on a warranty then the aftermarket ones are as good I would suggest as the BMW one. Mondial I am sure run most manufacturer warranty programmes (as they do BMW) when out of the normal 3 year cover.
Warranty Wise is the one recommended by Quentin WILSON
http://www.warrantywise.co.uk
Whichever way you go be sure on what they cover - ASK if they cover the roof motor in the event of failure. This is what is says on their website amongst other sections on excluded parts:
'Any Parts which are rusty, corroded or seized-up (such as catalytic convertors), Parts blocked up with carbon (such as EGR valves and DPF filters). Parts which have suffered Breakdown or Failure Due to Wear and Tear due to flooding or water ingress, de-lamination, condensation, freezing, burning, melting; a lack of or incorrect coolant, lubricant or fuel; blockage due to swarf or sludge; Parts which are electrically overloaded due to incorrect use of welding, starting or charging equipment; any fuel, fluid, coolant or oil leak not specifically included within Your Plan.'
http://www.warrantywise.co.uk/whats-not-covered/
The section on damage caused to parts by water ingress (irrespective of a design fault or failure) would appear to exclude the roof motor. But as above ASK b before you purchase.
