Traders who sell many cars from their house are still controlled by laws that apply to garages. Sold as seen does not mean anything, if the goods are not fit for purpose then tell him this and to repair it because selling a car that is illegal to drive, (because of the speedo fault) is illegal, let him know you will get in touch with the authorities, ie. trading standards, police, etc, if he won't help.
For the immobiliser fault see here...........
http://www.renaultforums.co.uk/search-148318.h tml