Is this the B8 platform? If so then I cannot agree that spark plug, coil-pack or injector failure is any more common than on any of the other Audi platforms. Whilst it is possible that the issue is with an injector, it could be something more serious including an intercooler leak. Injectors are not so hard to replace that the seller has an excuse for not fixing it before sale. I also would advise not buying in its present state. If the seller agrees to have the injector replaced first then maybe you could consider it but you'd be wise to obtain proof (in the form of diagnostic logs) that it fixed the issue.