How would you choose a writer if producing specialized material i.e. medical, industrial etc.

Most writers are versatile.
Research is a part of their job description.
However if the material very technical such as medical pamphlets it is necessary that the writer has previous specialized experience such as medical transcription etc.

Specialised material, such as medical/pharmaceutical or anything that is B2B-intended requires a thorough understanding of the subject matter in order for a writer to produce credible copy.
Therefore, it would be prudent to seek a writer who has a background in the particular topic, as they will better understand the subject.