However, the service provided as part of the subscription, a static IP address forwarding to your device without terminating the TLS connection is actually cool. For about 30$/year (24$ sounds good), I'd absolutely buy that alone, since that's the key to the tag line - truly private e-mail.
I can build a home server myself (and in fact already have, with more features, though not nearly as elegant as a Helm). I'd be just fine with someone else running that service.
Hardware wise, besides the storage, Helm looks like a great little home server, especially for the (hopefully not subscription-subsidized ...) price. If it had slots for my own drives I'd seriously consider buying one. The yocto-based software is a bit of a downside, as I'd prefer something like a Debian for maintenance reasons, but setting that up manually would sure be possible.