Doesn't the school issue one?
I would not recommend buying one unless you are totally sure that your DC needs one - usually the schools will give them out if they think children need them. If you are really determined to get one, find out from the school exactly what syllabus they are using.
There's no particular reason for there to be an "exact" syllabus at KS3, nor any reason for it not to change at short notice.
Quite possibly, but I have yet to work in a school which does not have a particular scheme of work and use one core set of text books in KS3 Science. I have taught KS3 Science to a range of children from different schools because of the nature of my work, and very little of my lesson prep has been transferable from one school to another.