If you have any uncertainty about the maintenance history of the car, or if the car spent long periods of time sitting idle, then it's a no-brainer. Especially considering you only paid a few quid for the car.
These engines like to driven often and for long stints. A regiment of short trips or infrequent drives is not good for the car. Moisture builds up inside and turns the oil acidic which wears at the internals.
I'd have the waterpump and IMS replaced soon if both are on their factory originals and you intend to own the car for a few years. Also replace the coolant cap to the latest version (important) and set aside some money for AOS, a common repair. All the above mentioned are the parts that can take down the engine. Any other repairs simply affect drive-ability. If you didn't have a pre-purchase inspection done before buying I would recommend doing a leak down
and compression test first to see if the engine is worthy of some big ticket repairs like the IMS. Also check the oil pan to see if you spot any glittery mettalics.
Are Boxsters this cheap in the UK generally? That's definitely a low price here in the U.S. for a 2.7 with only 80K miles.