Have a good look at the gearbox - chances are it will have dropped out/died already and they will have had a new one in, so thats one less thing to worry about.
Not a massive fan of discos, but I do have a defender with the same engine and they are, IMO the best engine that Land Rover have made, 200TD lacked some oomph, but Id rather have either of those than a TD5.
If/when the chassis is shagged you will be able to sell the engine - people are always looking for those and they pay good money - hell, Id probably buy it off you for spares.
The good thing about Land Rovers generally is that parts are cheap - filters for that engine are literally a few quid online, so you can service them for peanuts.