Whatever you choose, don't choose a pre Euro 4 Mazda diesel. The DPF is not fit for purpose and has a limited life. I speak with experience of a Mazda 3 that has cost and extra £1000 the day after it was bought for a non Mazda garage to try and refresh it but eventually replace it. Andy
they also run big end bearings due to oil contamination due to the way the engine management attempts to regen the dpf. mazda diesels of that era suck.
Depending on the driving style of the previous owner(s), your son needs to start saving for a DPF refresh which costs anything between £60 and £80 from a non franchised workshop unless there is a bill showing it's already done at about 80,000 miles. In the longer term (120,000 miles ish) the DPF will die and an after market part costs about £500 + fitting if he's not spanner competent. Buying second hand is fraught because you'll never know the true mileage. The DPF also has 3 different sensors screwed into it and the centre temperature sensor is renowned for welding into the thread and this is a Mazda only part which is very expensive but might be available second hand. He might just be lucky and not have to do anything before he moves the car on which means he'll have a little pot of extra cash to play with. Andy
My eldest has a T reg fiat punto , and my middle lad has a Pinto T jet 2009. I bloody love driving them very refreshing after the BMW,T jet is a super yet very sensible car.