re: ESA should just buy and fly an MER...
Buying an MER would be a lovely idea I'm sure...
But it's completely out of the question.
1) American export laws dont allow export of that sort of technology
2) European taxes pay for ESA. This money should then be spent within ESA - not going out to America. The only real way that American and European politicians can get away with the ammount of space funding they do is because it's money that is spent on american / europeans jobs respectively.
FWIW - I think exomars would be an enormous and costly mistake. We should be focusing on doing something that NASA seems to be ignoring - the Netlander concept.
Doug