Which Mercedes GLC diesel engine is best?

The 2.0 OM654 from the 2019 facelift onwards is smoother, quieter and cleaner than the earlier 2.1 OM651. The 220 d is the balanced choice; the 300 d adds noticeably more pull for towing.

What should I check on a used Mercedes GLC 220 d?

Look at the AdBlue system and the particulate filter, both of which suffer when the car only does short trips. On pre-2019 cars, ask whether the EGR valve has been replaced.

Is a diesel Mercedes GLC still worth buying?

For high motorway mileage, yes: it uses less fuel than the petrol GLC and tows better. For mostly short trips a plug-in hybrid suits better, if you can charge at home.

Cars

Used diesel Mercedes GLC in Portugal

Found 302 cars

From €20,950 to €77,900, half under €36,990 · Typically 2019 with 131,960 km. Diesel remains the long-distance GLC. Until the 2019 facelift the 220 d and 250 d used the 2.1 OM651; afterwards the 200 d, 220 d and 300 d moved to the quieter, cleaner 2.0 OM654, which the X254 keeps with mild-hybrid help.