£1k?pretty much dogend territory there fella.
The old 2.5 is bulletproof,I know that's an overused saying but it is,indestructible in normal use.
Rot is the killer,and everywhere.I'd put money on it that a mint old model transit would fetch more than a roughish millenium version.
I don't think ive ever seen a newer diesel one that hasn't had some sort of engine or sensor issue although there are plenty of folk out there who know their onions with these babys!!!
I can see from one of your previous posts that you use the van for a bit of work,you may find it better to look for an early vw t4,preferably a turbo model as they go really well,look cool and don't rust anything like as bad as either transit,although they don't have the headroom of the tranny.
Sprinters are good,but out of your price range or battered at a grand.
A good van for the money is an iveco,very reliable,big,strong and cheap for bits too.loads for a grand too.