Derived length and nildecomposable Lie algebras

We study the minimal dimension of solvable and nilpotent Lie algebras over a field of characteristic zero with given derived length $k$. This is motivated by questions on nildecomposable Lie algebras $\Lg=\La+\Lb$, arising in the context of post-Lie algebra structures. The question is, how the derived length of $\Lg$ can be estimated in terms of the derived length and nilpotency classes of the two nilpotent subalgebras $\La$ and $\Lb$.