Red interiors always sell slowly. Even without that, it's probably a touch expensive and that's why it's still there. I personally like it though. At that mileage, you'd want to see evidence of some suspension refresh, new coolant pipes etc.
I would personally not go with one with aftermarket exhaust. Mane sure you check that one for drone on motorway in 6th around 2500-3000rpm. Best ones have the stock exhaust with Porsche factory PSE or a mod called 'Carnewall' (done by a lad in Belguim on an exchange basis).
I don't see PASM as a big selling feature especially for a Batman on 17" rims as it just ads cost to the inevitable suspension refurbishment.
Is £16k your top budget? If I was you, I'd be trying to find the extra cash and finding a 2.9 gen 2 that was from 2009-2012. They have a lovely engine and whatever extra you pay for it now you will get back if you ever sell it. The Porsche market has cooled the last month or so.
BTW, all Caymans have the stronger IMS bearing and the 2009 onwards don't have them at all. It was only the first year release 987 shape Boxster in 2005 (released a year before Cayman) which had the weaker IMS that is normally changed during a new clutch install.