From what I understand you need an 11-plate or newer to have the revised tensioner fitted. It wouldn't put me off buying an 09 or 10plate car, just look for one thats had the updated tensioner fitted, or budget to get it updated to the latest spec (a few £hundred at a good independant garage, but much cheaper than a possible engine failure).
As always, buy on condition rather than mileage etc. Are there any options you consider a must?
Also, you can now check car's MOT history with just the reg no at
https://mot-history.net obviously looking for any fails/recurring advisories etc.