Drivable vehicle when purchased and undrivable when collected.

I purchased a 2006 Mini Cooper S that was a runner and drivable but with two disclosed defects, that of the convertible roof not retracting and the drivers door jammed shut, for the discounted price of R42 500.00 cash However, when arriving to collect the vehicle their staff drove the vehicle to their workshop to replace the battery, thereafter, it became immobilized for some unknown reason but suspected the on-board computer having blown. My complaint is that I purchased and paid for a car that was a drivable runner and as such I should be refunded or the vehicle replaced or the vehicle repaired by the dealership. Southgate Wheels insist I brought the vehicle with known defects and as such ignore my options, as a 72 year old pensioner buying the car for my Daughter, to settle this fairly.