There is several sets and each set has include different parts. For example Set 1 includes A, B; Set 2 includes A, B, C; Set 3 includes A, B, C, D.
As a result, it depends on whether you need to include all or just some of them. I suggest you find a consultant to know more detail as it is no obligation to buy.
Moreover, you can buy separately. Say if you buy Set 2 then later you want to have part D then you can buy part D separately. However, the cost is higher than that you buy Set 3.