Hello, I hope the following is of assistance.
The most important thing at that mileage is to make sure the timing belt and injection pump belt have been changed, they should be replaced at around 200000 KM. If there is no evidence these have been done take this into account for the price of the car..
Also check that there are no vibrations through the clutch pedal and rattles at idle that disappear when you press the clutch, this engine does have problems with the dual mass flywheel failing which is an expensive repair should it need replacement.
Look for any signs of diesel or black plastic around the injectors as the seals can fail on the injectors, you can usually hear a "chuffing" noise as the gases blow past the injector seal if this is the case.
Check for any sign of oil around the short pipe to the inlet manifold, this may indicate a split in the pipe.
Check the electrical systems all work OK as some Cmax suffered with water getting into the fuse box causing various problems, this tends to be on earlier models though, a 2007 should be OK.
You may read about EGR valve concerns but this tends to be the 1.6 and 2.0 diesel engines.
Other than these common concerns check the tyre wear as larger diameter tyres on the Cmax tend to wear quickly an unevenly if the wheel alignment is poor.
Start with these points but check the car as you would any car before buying.
Overall given the number of Cmax on the road they are a reliable car and the diesel engined cars will do many miles as long as they have been well maintained.
I hope this helps but if I can assist further let me know and I'll reply as soon as I'm able to.