14 miles each way will get the engine hot enough to regenerate the diesel particulate filter and with a regenerative alternator should recharge the battery. But these are some of the potential costs of an EU5 diesel between years three and six: Replacement diesel particulate filter: £500 - £1,000. Replacement dual mass flywheel: £1,000 - £1,500. Replacement EGR valve: £500 - £1,000. Replacement turbo: £1,500 -£2,000. Replacement timing belt and tensioner (where fitted): £500. All that adds up to considerably more than any saving on fuel even over 100,000 miles. The saving of 50mpg v/s 40mpg over 100,000 miles is about £2,500; the saving of 60mpg v/s 40mpg over 100,000 miles is about £6,000.

I'd be looking at a MINI (chain cam), an Audi A3 1.8TSI cabrio (chain cam) or BMW 1-Series cabrio (all chain cam).