Main things to check for on Classic are: rust, rust and rust (did I mention rust?)
Look at the front inner wings, sills, footwells, rear cross member, rear seat belt mounts in the wheel arches and the boot floor under the insulation. These are all well known rust spots, but can be prevented by care and attention and cured if caught early enough.
Also check for overheating/head gasket. If the temperature gauge goes over half-way under normal driving, then there's a problem with the radiator - they can silt up. Simple check is to feel the top and bottom hoses. If there's a big temperature different, then the rad is silting up
Check the viscous coupling. Simple way is to give it full lock on loose ground and see if the tyres scuff which could indicate that the coupling has seized.
Also check for damp carpet in the front footwells. If it smells of coolant, then the heater matrix is leaking, if it's plain water, then it's likely to be the door seals
Despite the above (which I've learnt the hard way with mine), they're an awesome motor and I wouldn't swap mine for anything. That said, remember it's a 15 year old vehicle and will need some TLC.
In the UK, I'd say that 4000 Euro is over the top. I'd look to be paying just over half that here for a good mint one