Things to consider with lease purchase as I have just found with extras is you need to find what the second hand values quoted in say Glass's guides includes. The values for the C2S for instance includes "sat Nav", so if you don't have it they will quote lower than book price.

Porsche in the UK generally quote a Residual value of 50% over 3 years and 45% over 4 years. If you push hard you will possibly get Porsche to agree to quote a 50% residual over 4 years.