Firstly - Western seem to be pretty good at dealing with customers and any problems that do arise.
Common issues for the 2.4 are:
sticking egr valve
inlet manifold (later models had a revised "perforated" manifold so try and find out what the car you are looking at is fitted with).
Early models had problems with the DPF blocking, but this was fixed with a reprogramme of the ecu, so I doubt it would affect a 57 car.
Creak from the windscreen - sounds like a cable moving about inside the dash. There is no permanent fix for this but there is an easy temporary fix using a silicon lube around the windscreen rubber. Just need to re-apply lube every three months or so.
Buzz from the rear view mirror - new mirror is the solution to this.
High fuel consumption (low 30's mpg) probably because of the DPF.
Steering fluid should be green in colour (early cars were fitted with a red fluid which isn't viscous enough). Western should be on top of this one.
HIgh inside edge tyre wear. Setting tracking to parallel fixes this. Again - check with Western that this has been done.
That's about it.
Hope you like the car - it's a great car to own, drive and be seen in