Hello,
Basically it doesn't come down to the shape or size of the vehicle but more the engine thats in it. The older diesel engines which are kicking about like the 2.5N/A (Non turbo) and the 2.5TD (Turbo diesel), although usually slightly cheaper aren't the best choice for the motorway. They barely reach 70mph and they certainly struggle on hills without any weight behind them, so I can't imagine what they'd be like pulling a trailer or a caravan up and down the A1 for example. They also drink more fuel and aren't considered to be as reliable as a few of the newer diesels.
The 200tdi (turbo diesel intercooled) is the engine that most people swear by. It came in around the early 1990's. It pulls like a train, it's quite economical and it's very reliable. If you can find yourself a 90 with a 200tdi engine, or even an older 90 that has had a 200tdi engine fitted you're laughing.
The early 300tdi's (slight modifications over the 200) are also considered great engines, but the later ones got a bit of that computer wizardry added to them which although doesn't cause any problems, you're better off without.
Another great engine which is now relatively cheap is the TD5 (Turbo diesel 5 cylinders) which is very quick, but also quite economical with plenty of power. Again, the whole engine is governed by electronics and they are easily the one most engine that people have the most problems with in terms of ECU bugger ups and stuff like that.
I'd say go late 200tdi, early 300tdi (it doesn't really matter) but just make sure that the chassis and the bulkhead are rot and rust free and that the engine isn't covered in oil.
-Pos