The 3.0 dci engine is a real can of worms, I personally would look elsewhere, if however you are adamant then the main things to check are ....
Cylinder liners: 3.0 dci's have bonded cylinder liners which are prone to becoming detached and dropping resulting in total engine loss. Do not consider one which has not had this sorted. Renault replaced about half of the engines under warranty, others were sorted by machining the block and fitting flanged liners, however this was never an official recall so not all have been sorted.
High pressure fuel pumps: known for premature failure of the solenoids and seals. Full repair involves replacing the complete fuel rail at circa £3500. Some people have negotiated a contribution from Renault, I had the pump reworked in situ for about £750.
valves. Poor system design leads to premature
valve failure. Blanking is the best option but this adds extra stress to the
replacement costs around £1000
: Variable vein control rings prone to sticking, and main shafts wear causing
seals to fail. Never good as this leads to the engine running on it's own
Automatic handbrakes prone to breaking cables. Cables alone cannot be changed so your looking at £600-700 repair.
Injectors prone to failure, cost around £1000 a set remanufactured.
All above are common faults on the 3.0dci
Digital dashboards, airbag systems, suspension failure, uneven brake and tyre wear are all issues associated with Espace IV in general