I take it you mean either the 530D or 535D.

Both are very capable cars, things to look out for are you really need to have full service history. If its mileage is circa 120K then factor either a DPF change at £1200 or a DPF removal and remap at around £700. Also they are prone to loosing swirl flaps from the manifold that chew their way through the engine and turbos. These can be removed and blanked with very few issues for around £50 for the parts and a couple of hours work.

They also have vacuum operated turbo actuators whose pipes run along the monifold on the drivers side and can cause issues and power losses. These only cost about £30 for the pipe and again a few hours work to change them all, best to do the whilst the manifold is off doing the swirl flaps. The EGR valve is also prone to sticking but easy to clean.

Check the cars temps are running correct at around 90 degrees as otherwise they are thristy but the main culprits are the egr thrmo and main thermo which are around £40 and £80 each.

Aprt from that brilliant cars, the 530D can be mapped to around 280 bhp almost the same as a standard 535D. However the 535D can be mapped to around 350bhp and over 500lb ft torque which makes them quite poky!!!!!