One of the reasons for Clinton having her own email server is to securely delete any email(s) that she wants to.
Of course, it is still possible that the hoped-for secure delete is not as secure as imagined.
And also, of course, any email that she sent presumably has a copy or copies at the recipient’s mail server, and in some NSA database, and probably also in some KGB database, and maybe elsewhere. But those won’t fall into your lap just by seizing her server.
If you can replace the drive, then it’s secure.