Hi Antics and welcome

£6000 should get you an 07 plate at about 20 - 25 thousand miles old, possibly less. As mentioned before there are plenty out there so have fun looking and normally you will know when you know you have found the `one` for you!
The ST is a good choice, I have had my one for 4 years now and it has been fun, reliable and after all this time I still can't help but smile when I put my foot down hard

Apart from the obvious things, check that the gearbox is nice and smooth (not like the one you tested before) and that you don't find difficulty in selecting any gears. This can often be a sign of wear and tear and also that it has been ragged quite a bit.
If test driving it then listen for any engine rattles or `pinking` when driving above 4,000rpm. Don't worry about cabin rattles, from what I know there are very few ST's out there that don't have a few loose screws

Check that the car doesn't pull to one side too drastically (slight left pull is not uncommon) Check tyres for uneven wear that may indicate tracking problems.
If you can, check the heated front screen works (a pain to replace at your own expense)
The engines are pretty durable and unless they have been really mis-treated, you will not find too much goes wrong there.
Good luck and let us know how you get on.