@exilit One test report should cover it. I say should as it is really up to the Certifying Body to determine it any further reports are necessary. These could be FCC (radio frequency reports), LVD, CISPAR or statements that report the use of certain products used in the manufacture and disposal of the PCB, Components, Solders etc.
Certifying is a very comprehensive process and I think that any advice I give you really is only an opinion as requirements will vary.
My feeling is that you should obtain professional advice from a Electronics Engineer who has all the answers to the questions you ask before proceeding any further.
In answer to your points, you should be able to obtain certification on a device that has an additional interface, however, the device that interfaces may also need certification or may already have it.
Doing it yourself I meant as a a DIY project, oddly if you offer it as such then there appears to be no obligation on your part to seek reporting or certifying advice.
Sell it as a kit and buyer beware.
