Hi Indigotech,
The Exchange Plan 1 includes organization-wide capabilities such as journaling, transport rules, and premier anti-spam and antivirus filtering via Forefront Online Protection for Exchange. You can download Microsoft Exchange Online for Enterprises Service Description.docx and Microsoft Exchange Online Archiving Service Description.docx to see additional information about this.
However, for the question about using third-party solutions, we cannot provide the real answer. We welcome community members who have experience of this to sharing the helpful information with us.
Thanks,
Maggie Li