I've had a 2003 Td4 for 5 years now and it's just turned over 70k miles and tbh it's been very reliable. Barring a couple of failed switches (heated seat and headlight leveller, which were replaced with items from eBay) the only problem has been a failed injector.
It's mainly used for lugging stuff around in which I can't fit in anything else, supermarket runs and in $hitty Winter weather. It's never going to be fast but it's not mindbendingly slow either and wobbles round the corners abit but it is what it is, serves a purpose and returns around 35/mpg. No point spending a pile of cash on anything else just to use as a workhorse imo.
I went for the ES which has a few more creature comforts, full leather, heated seats, aircon etc etc.

I ran an 03 ES TD4 for four years. Average fuel economy was circa 30 mpg. Mine had the BMW engine which I believe is the better one.

Common expensive failures are the sunroof mechanism and the rear window controller (the rear window has to drop an inch to open the rear door). Both £500+ parts alone. Other less expensive known issues involve the fuel pump. Also some switches (and particularly the downhill descent sensor) as previously stated.

When sold it had just about 100,000 miles on the clock and the auto box was becoming a tadd clunky in selection.

Be aware the load space is not very big as the seats do not lie flat and when the seats are raised it is not suitable for much either.

Would I buy another ? Yes, probably. Comfortable on long journeys and with the right tyres, road noise is greatly reduced.