You want to pay somewhere between £350-£500 depending on warranty, make, model and date of purchase.
Blower types = £350-£380
basic card with basic after market cooler £380-£400
more top end models £400-£500 depending on how much warranty left.
EVGA king pin edition will go for £550-£600 because they are the best cards.
dont worry about buying a mining card if it has good amount of warranty remaining and from EVGA ideally or the rest i mention below.
I would personally look for EVGA, GIGABYTE , ASUS and MSI only because these companies allow 2nd buyer to claim warranty, ie claim warranty without receipt. EVGA now want proof of purchase by second buyer after march 2018, so double check. The rest will do warranty from date of manufacture of the card so determining warranty could be tricky. Even if you have receipt they will not service warranty from date of purchase as that is only valid for original purchaser of the product not you. Apart from EVGA that is, they are happy to give you warranty from either date of manufacture or purchase if it was registered by the original owner but in order for them to give you warranty, they want to see proof that you bought the card of this person as far as i understand.
STAY away from PALIT, ZOTAC and the rest as their warranty is through the shop where it was purchased and they will only deal with the original purchaser, unless you are willing to impersonate that person( which is illegal) or you are come to some kind or warranty arrangement with the person you purchase the card off. Palit make great cards but bad warranty. ZOTAC if my memory is correct is either 1 or 2 years through point of purchase and the remaining warranty through them online.
best way to find date of manufacture is to get the serial number and fire email to each respective company and ask them, they will tell you.
good luck.
