It's not that often that a decent exhaust system comes up for sale on here and when they do they're usually snapped up pretty quick. That Milltek is the best part of £900 new from most companies AFAIK so it is a fair price for a used item.
If I was you I'd really try to get that Milltek (put it on a credit card if you have one maybe?) otherwise you're going to end up spending £300 on a "Crapspeed" exhaust (I saw your other thread) which will either not fit properly, be too loud, need welding/replacing in no time or just really not be what you want.
The other option though if you really can't stretch to that used exhaust is the Cobra exhaust system which is around £450 odd new for a centre section & back box.
Definitely worth the extra cash for a decent system imo if you can stretch to it or can wait to save up that little bit more.