Is it appropriate if my former advisor is editor of a journal I submit to? I am a young postdoc. I am considering submitting a single-authored paper in a journal. My former advisor is member of the editorial board. Would this be appropriate? Obviously she cannot be the handling editor, someone else would have to manage my submission and take the decision. But would it be appropriate at all for me to submit in this journal, or should I just find another?

I am afraid that this situation does not pass the "appearance of impartiality" test; even if my former advisor is never involved in the decision, if the paper is accepted in the end my situation will be "I have published a paper in a journal where my advisor is member of the editorial board", and no one will be able to verify that I did not receive unequal treatment.
I don't see any conflict of interest on your side preventing you from submitting to this journal. You can only assume that your former advisor has good ethics and will abstain from handling this manuscript.

Consider the consequences if someone you had a close working relationship with being on the editorial board would prevent you from submitting to a journal: many senior scientists could not submit to any journal in their field.