Is it a good idea to buy a used diesel now?

I generally buy a used car with around 20k miles and run it for 6/7 years before replacing. At the moment I'm looking at buying a Mercedes E220d estate for around £20k. Is this a really bad idea? The general...

What's a good diesel to buy on a £3000 budget?

I'd like to buy a used diesel family car within the £3000 limit. I need it for commuting so needs to be good on fuel? Any ideas and what mileage is best for the age?