I have a 2006 One D. Have owned it just under a year but the only I've had (and the only issue recorded) is the replacement alternator two months ago. Wouldn't worry about engine cover not an essential part of vehicle engineering!
It does very well on motorways in the UK and sips fuel
I recorded 85mpg on a motorway drive to Southampton last weekend!!!
Plenty of punch for someone my age and has all the toys equivalent of a Cooper. IMO they are a brilliant buy!!