with gas at 54p a ltr i have no problems with the running price,its just that the wife just does not like.I dont drive as i ride motorbikes so its her vehicle not mine.
If its not running cost's then stay with your disco and run on petrol so no miss fire, you will also still have a bit more power than a Td5.
TG5's are very good for towing and petrol good for speed and as for the model it has to bet he last of the ES or ES premium
As its for the wife a 300tdi is a brilliant work horse if your lady likes a bit of comfort go for a good td5es great for towing and with a few mods not much behind the v8 for performance
Not sure about the ES though, I run the base model TD5, you still get air-con and the towing ability but with a lot less to go wrong, no ACE, no air springs, no sun-rooves; I guess it depends on your (wifes) priorities? have fun...
Keep the V8 and get the backfire sorted. Change ignition leads, spark plugs - get the system serviced. I have two V8's both with Gas (one sequential - one single point. Never get a backfire