Yes, you can use a booster in the front seat, in fact you are required to if they are of the height/weight where they would have to use one anyway. If they are in the booster then I would also leave the airbag turned on.
Personally speaking, With 4 young kids, I would go for the biggest car I could get.
The Grand Scenic will have 3 ISOFIX points in the middle bench, and 2 extra seats for occassional use, or a large boot which would come in very handy.
For modern cars, I do not think there is much of a muchness in reliability. All modern cars, especially diesel, are complicated now and all can suffer from similar expensive repairs.
If you research the car/engine you have you can usually figure out what the major failure items are and even nip them in the bud, preventative maintenance.
I used to own a 10D Grand Scenic 1.5 diesel and I really liked it.
If you can, make sure your budget covers a suitable car and still allows you to put 1k aside just in case something major goes wrong, and then don't worry about it.