MadDave
Depends how rushed you are to get one. We ended up driving to Gloucestershire from Sussex to buy ours.
You should be able to get a reasonable beast for your £4K, but it will take a lot of looking at duff ones before you find the right one.
Rust will be your enemy, and make sure you give the chassis and rear crossmember a good tapping with a hammer. These will only take so much patching, and you want to make sure that you dont get one that you will have to rebuild. Waxoyl is great to see, but I would be wary if it looks like it has not long been applied.
Front bulkhead is a must for checking - although you can get kits for repairing sections of them.
90's are more expensive, but you pays your money and takes your choice. Try to get one that has not really done the off-road day thing.
Are you looking to do some off-roading?
How many people are you planning to take with you?
Would you consider buying a van version and getting some windows put in (combine this with a trip to the Old Sodbury Sortout) and you could upgrade from van to uncomfortable people carrier for less than £300. (helsp you see properly at junctions too)
I remember seeing some low mileage 90 200TDi's online, and for less than £4K - but they go pretty quick.
Good luck with the hunting - make sure you keep us informed. Land Rovers are important