I'm looking at buying at Merea weekend TD 100 ELX for £589. Its got 180,000 and service history It looks like its in good condition. Will this still be an ok car? Or will it be nearly dead at 180,000?

If it's been looked after and well maintained it could still be a very reliable car.
Only way to tell will be to have a good inspection of it.

at this mileage you need to check the history carefully. anything on the steering/suspension/brakes that has not been changed will almost certainly need replacing by now. same goes for all servicables and a lot of parts that you would not normally consider servicables, such as engine and gearbox mounts.

obviously it depends greatly on where it has been driven, and how it has been driven. an abused car with 80k will easily be worse than a cared for car at 180k.

it would be great if you could check the engine compression before buying. by this mileage some engines are still good as new (if it was a merc for example ) but many have lost so much compresison that the performance and fuel economy suffers significantly.