Highest priority: status visibility
The single most common reason clients email or message you is "where are things at?" — a self-serve status view eliminates the majority of these messages on its own, before any other feature is added.
Second priority: document access
A single, findable place for documents (contracts, deliverables, invoices) beats a scattered email-attachment history — this is usually the second highest-value addition.
Lower priority, usually: in-portal messaging
Full in-portal messaging (replacing email/WhatsApp entirely) is a bigger lift and usually less urgent than the two above — many clients are fine continuing to message through their existing channel once they can self-serve status and documents.
Billing and payment links
Valuable, but usually not the first thing to build unless payment collection is currently a specific, named pain point — see How to Plan Roles and Permissions in a Web Application for how billing-related access needs its own careful scoping.
Questions
Should we build everything in one release?+
Usually not — status visibility and documents alone solve most of the pain, and later phases can be scoped based on real usage.
Does messaging replace our existing communication channel entirely?+
Not necessarily on day one — many portals coexist with email/WhatsApp initially and expand into full messaging later if there's real demand.
How do we decide our own priority order?+
Look at what currently generates the most client-initiated 'just checking in' messages — that's usually the highest-value first feature.