I haven't exactly tested mine off road ........
But, what I will say is that the Discovery is dead easy to work on, like a big Meccano set, 99% of parts are available second hand, and are dead cheap, new parts such as service items are available very cheap, full filter kit is around £15 off ebay.
There are loads of lads out there who are happy to give good free advice and help, mine is a TD5 and I've done loads of work on it to get it looking and driving like a much newer model, I haven't found a anything yet that someone else hasn't done to them and is willing to give advice on.