We'd say that 15,000 miles a year warrant a diesel, particularly if you don't cover many town miles as the DPF needs to get hot enough to burn off the soot that'll block it. That said, you don't say the age of the vehicle in question. If it's an older diesel, I'd certainly be concerned for two reasons. The first being that older diesels are being banned in city centres and this is only going to get more widespread as time goes on - so you may find you have to pay clean air charges in certain places. The second, more serious concern is that the Insignia has only done 41,000 miles. If it's done this over 3 or 4 years, that's okay. But if it's a 2015 model, for example, it's done less than 10,000 miles per year. A diesel needs to cover enough highspeed miles to ensure the DPF doesn't get clogged up and fail. It's expensive to fix/replace a DPF so the low price could be due to the fact that it's nearing a large repair bill. I would speak to the owner about how it's been used and look at any failures and/or advisories, plus the recorded mileage, on the MoT history. You don't want to buy an older diesel and get stuck with issues as soon as you buy it.