There are two diesel engines. The earlier L-series Rover engine (68bhp) and the later TD4 BMW engine (110bhp).
The changeover was at the 2001 model year (2001MY), which was in 2000. So basically X and after are TD4s.
The L-series has a reputation as a bombproof, reliable engine. They were the used in the Montego car, which whilst in itself is not much to go on, remember that they were popular for minicabs and taxis because you could get 250,000m out of them without raising an eyebrow.
The TD4 is more exciting and is the same engine as the BMW 3 series. It also has a reputation for reliability. The one characteristic known problem of any note is the Mass Air Flow (MAF) failure which is easily recoignised and simple to fix. See
Tuning-diesels for cheaper diesel tuning. for more info on that).
You have already determined that petrol Freelanders are not a safe buy, unless you are a keen enthusiast and/or amateur mechanic. There are people who swear by them, but the risk of Head Gasket Failure is enough to put most people off.
When selecting a Freelander you also need to throughly check the transmission. You need to ensure the VCU is not seized and there there are no thumps or bumps or other funny noises from the vehicle's drivetrain. If you do detect problems - don't panic - just return to the forum for a diagnosis. Reading the forum, there are many cases of "terminal failures" being resolved with a £20 part!
To summarise, there are pit falls in buying a Freelander, but get a good one and you will have as fine a car as you could want. The difficult thing is finding one that you are confident enough to buy.
Cheers
Blippie