It is always a difficult situation to be in......
Do you either spend money on it and keep it,and hopefully have a few years of problem free motoring,or do you put that money towards towards another car that could be just as troublesome.
If you've lost confidence in the car and are fed up with it,then I'd trade it in and buy something else from a dealer with a decent warranty on it.
If you still like the car,then spend the cash thats needed to get it to a condition where hopefully that will be the end of the big bills for the foreseeable future.
I know it's very difficult,and I don't know your financial position,but I'd be tempted to spend the cash on your Scenic and keep it.Any car needs tyres,brakes,cambelts etc,so these are just maintenance costs.
Just one point though,if your car hasn't had a cambelt change from new,then it's well over due.They need to be changed every 72k or 5 years,so yours has done well to last this long!
Let us know what you decide,and the best of luck to you in whichever decision you come to.