Oh dear, not another Chris on this site... Welcome to the club btw.

As for your questions:

As far as I know, there's no way to keep the existing 6 CD changer with a new headunit. The standard CD changer is an in-dash double size unit, not a boot/glovebox changer like you get on most other cars. If you wanted to change the headunit then you'd need to get a CD changer to go with it.

The ST has part-leather but if you pay for the full leather you get heated seats too. There's a simple on/off switch (with a timer) which is located on the base of the seat - a bit fiddly IMO. I've not used them yet as its not been cold enough. If you can afford it then I'd recommend getting full leather, it looks saweeeeeeeet.

I did a 0-60 sprint at Silverstone the other weekend, got 8.76 seconds. Not spectacular, but then I've never done it before. Some magazines have said that the quoted figure (7.9) is unrealistic, but I know of guys with standard cars who have done it in 7.2 seconds.

I wouldn't bother with ESP if I were you. The ST isn't massively powerful anyway, and in the dry the Pirellis offer more than enough grip. ESP costs £350 and as Chris says, I believe you need to get the Reflex pack as well to get ESP. That's another £275. I haven't driven one with ESP yet so can't really comment though, but I would say its not necessary. The only reason a lot of manufacturers sell cars with some sort of stability control is to cover up a crap chassis anyway...

You don't notice any loss in performance from having 17s on the car, tbh but I've not tried an ST with the 16s (you can't get them in the UK). The car looks a billion times better with the 17s anyway.

Not much is out in the way of tuning parts for the ST yet. There's a few exhaust systems about, the Milltek is favoured at the moment, the full system with de-cat being proven to give a 21bhp increase. Induction kits will be out soon enough, but you'll want an enclosed one due to the cramped engine bay allowing too much heat to get in. Superchips hope to have a Bluefin out shortly too (that's what they told me at Ford Fair). I reckon with a proper induction kit, full exhaust and chip/remap, you could easily see an extra 30bhp, if not more.

Anyway, hope that helps Tyson, sorry for the lengthy reply!