The nut is available from Renault as part of a fan blade fitting kit. Part no - 7701205080.
In fairness it is expensive as I don't recall needing anything else but the nut and a washer. Think the kit was about £4. The blade is about £30 !!
As for a fan motor, don't quote me so do you homework first !, but this looks about right. Personally I would go for new as in my experience the wires tend to go brittle where they enter the body of the motor.
http://www.ebay.co.uk/itm/RENAULT-5-...item35af9ef994