I'll explain why I spent my money on a Corsa VXR rather than an Evo, M3, TVR, 996 or any of the countless other cars I could have had for the same money.
1. Warranty - Its highly unlikely that I'll incur any repair costs inside the three year warranty.
2. Reliability - A modern car is nearly always going to be more reliable
3. Running costs - Any of the "good" second hand cars you mention will attract at least three times the running costs, almost guaranteed. I average around 35MPG.
4. Servicing - 4 pretty much standard plugs, 5 litres of gm oil, cheap paper element filter, all easily accessible, and even a dealer service can be had for around £120.
5. The feel of a new car - no matter how well a car has been maintained, by the time cars get to around six years old, they just don't feel the same any more.
6. £550/yr to insure, fully comp - the same as my old Corrado VR6 and ACS 325 cost me TPFT - to put that in perspective, I cannot insure an Impreza, Evo, M3 or 996 for under £1300.
And about a million other reasons that I cannot list right now.
Even past the warranty, an engine rebuild is in the region of around £1200, turbo and manifold is £500, wheel bearings, ~£30, pads are no more expensive than any bog standard pad, discs aren't horrendous either - everything is cheap.
You'd have to be a sub89er to be so blind that you cannot see past the initial purchase price.
You should be glad you didn't try and buy a 996 for 15K, it would have been 3 different cars glued together for that money unless you were very lucky...when it would have been only 2 glued together, with some tape.