If i was looking for a MINI and found the one you found, here is what I have to say:
If it is Cooper S, this is a great opportunity for a great price,, "BUT",, you must check exactlly why is the seller selling it, and to be more confident, ask to take it to the dealership for an A-Z check up or ask them for its maintainance history.
If it was a Regular Cooper, you might wanna pass by MINI Dealership and check what they have to offer you and through in a word or two stating "Ya I found a cooper at price of 21 000, how much could you do for the price to make me want yours instead !!" If you argue with them about a Pre-Owned MINI,, try to metion "CASH".... I will give you a deposit cash today or tomorrow if you accept my offer !!
Try it as well with the seller, he might turn it down,, give him a day or two and chances are he will call back giving you a better price.
If it was a Classic Cooper ! Forget it, its a rip-off.
Hope these advices are helpfull to you.
Let us know about your decision