We had a 03 plate Cooper S for from new for 3 years. They are fun cars, but completely impractical if you intend to use it for a mode of transport to do any decent mileage or if you intend to have anymore than one passenger or carry any loads.
It got some hard use, but in the time we had it the PAS motor failed and the loom powering the motor had a electrical fry up, it pop'ed two heater matrix's, supercharger belt tensioner failed and gearbox was replaced as lathe shaft bearings were clattering away.
MPG is not great 33mpg was the best we achieved. If you accept that you are going to be filling it regularly with fuel and fixing niggle problems then you can appreciate the car for its fun factor. You have to screw the engine to the redline to get the car hauling at any pace as they really don't feel that torquie compared to other superchargered cars. What they lack in engine grunt they more than make up for in their ability to out handle other vehicles and carry momentum along twisty roads and the brakes inspire confidence. The brake system is exceptionally powerful whist retaining exceptionally good pedal feel and progression.
Jesus sounds like you got a lemon.
My mini adventure is completely opposite; had mine nearly 4 1/2 years no problems, I did wait till the facelift model before purchasing one.
The petrol tank could be bigger; it's a pain in the arse on long journeys.
I would try and get a late Cooper S\JCW, after Jan 05 and before the turbo. You get a different supercharger to the pre facelift and LSD was fitted as standard. Also with a factory built JCW, they started fitting the JCW brakes as standard.