Check out contributions from various forum users and their repos / offerings. That will ensure capability to some extent. Ethics, you will have to play by ear and experience in an ongoing support relationship.
Since you want a retainer based consultant to maintain the installation, specify the platform (Windows, Mac or Linux) and the kind of server (apache/IIS/nginx, etc) and PHP/MySQL version familiarity.
To check for interest in assisting you, quote a compensation value affordable to you and any language preferences especially if other than English and any specific accounting knowledge you will need to depend on for operations and understanding. TimeZone issues are on the edges only.
Arm yourself with sufficient FA knowledge so that you can migrate between consultants easily without vendor lockin.