To be quite honest I'd say don't buy it!!! That's only because you're my brother so I won't be able to make a good margin on it and I need the pennies for the GT-R
Looking at similar models (Cayman S 2006, 55k to 85k miles) with no engine rebuilt (so chances to have bore scoring are as big as they were on my car) you can see them going from £15k upwards
http://www.pistonheads.com/classifieds/used-cars/porsche/cayman/porsche-cayman-24v-s-2006/4384209
http://www.pistonheads.com/classifieds/used-cars/porsche/cayman/porsche-cayman-3-4s/4523981
http://www.autotrader.co.uk/classified/advert/201507145192224
http://www.autotrader.co.uk/classified/advert/201406124962712
http://www.autotrader.co.uk/classified/advert/201506114239033
http://www.autotrader.co.uk/classified/advert/20150802571360
http://www.autotrader.co.uk/classified/advert/201508105922873
and so on.
Hard to put a price on mine, what do you guys think it will be a fair price, considering the brand new engine and 1 year warranty which is usually worth about £1k?