The only time I would consider part worn is if they were winter tyres. At least in that case they're unlikely to have gone on a track, or driven hard, and physical damage is more likely to be 10mph kerbing rather than anything more serious. And the nature of winter tyres means that there is a reason they would be taken off before they wore out.
I agree with Camskill or even the more mainstream ones like Blackcircles, 225/45/17 is a very common tyre size and there is loads of choice. As long as you stay away from the absolute bottom of the range, you should get yourself a decent tyre.