Not a bad option if you can get a diesel for the right price. Petrol don't even bother, you'll need to be an oil baron to run it (I have the 3.5 v8 Range Rover Classic).
But the TD5 is apparently bulletproof, it'll just been the rest of the car that lets it down (electrics not well known for reliability!)
To be honest if the plug itself has never been changed then it can't hurt to change that too. They too can suffer thread damage and eventually fail from being overtorqued during most oil changes.
Car: 2002 Forester XT (2.0)
Tuned: Stock
Fuel: Z 95
Fuel Economy: Just under 10l/100km so far, mostly long trips. Will see in time how bad it gets in town!