It really depends on what he is earning in the UK as to whether the basic salary is reasonable, he should negotiate for a basic salary that is the same as what he is earning gross a month at home, then look at the benefits on top of that. When you say schooling is covered, is that up to a certain limit per child? The school fees vary significently from school to school.
I am a bit confused about the comment about seperate accommodation for your Mother in Law...is she coming to Qatar also? Who would sponsor her for residency, as I do not think your husband would be able to sponsor her on a family visa. Maybe someone else here knows more on that subject..best you check.
Cheers