The Camshaft issue affects a known vin range, see here
http://mazdacx5forums.co.uk/index.php?topic=996.msg6675#msg6675Any car up to that number can be affected at some point. Mine went when my 2012 sport nav was only 18 months old at 30k miles. Some are still happening now at 100k miles and more. Best to avoid any affected car unless you can confirm that the cams have been replaced and any associated work, usually turbo, has been sorted.
Any car after that VIN should not be affected, the problem was due to incorrectly hardened cams.
If the diesel is used for a lot of short runs, the DPF has to be regenerated more frequently, that burns diesel in the DPF to clean it. Unused diesel from that process is flushed back into the sump, diluting the oil and raising the level. More frequent oil changes are advisable in that case. Late 2018 models with Adblu don’t have this issue.
Mazda never officially acknowledged the camshaft problem, there was no recall. It was dealt with on a “sort it if it happens” basis rather than being proactive about it. It’s quite possible that some dealers have never seen it happen.
Consider the usage; short trips, low usage best suits the petrol engine. You want something to munch miles and drive hard, get a diesel. Whichever, ensure it has a full Mazda service history and get a good warranty.
Alan