Right ok. 2009 Fiesta titaniums are currently going for 5-5.5k on autotrader.
Taking a 2014 St-3, it is reasonable to think that it will probably be worth a bit more than that but lets say as a rough figure that in 5 years time Fiesta ST-3 is worth £6k. Depreciation then of £10k ( assuming a purchase price of £16k)
2002 Porsche Boxsters seem to be going for about 6k on autotrader. So in 5 years time, your 13k 2007 Boxster will be worth around £6k. Depreciation of £7k.
That is only £3k more depreciation from the Fiesta.
So using that £3k figure of the extra running costs per year, you would be £12k better off after 5 years with the Fiesta ST. Also, that is assuming the Porsche doesn't throw any horrible big bills when it starts to get old and cant have the Porsche warranty put with it anymore....
You can extend the warranty on the Fiesta St when new to 5 years and 100,000 miles for a one off £240 when you buy it...
I am sorry but if you are actually going to use the car a lot (i.e. do 10k+ miles a year in it) and you want to be able to keep it for a reasonable amount of time, there is no way a Boxster S is going to be cheaper or cost the same money overall when compared with a new Fiesta.
Also, what does the 2004 being a different model have to do with anything? The 2007 will be considered "an older model" at some point down the line when Porsche release new ones so I don't see how that has any merit?