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.
A requirements checklist before you build
Before committing to features, write the answers to three groups of questions. Roles: who the portal users are, and what each may see and do. Documents: which documents clients need, who uploads them and how they are named and found. Notifications: which events should tell a client something, and by which channel. Then test the result with real users; see client portal acceptance testing.
Decide early whether an account belongs to a company or to an individual, or both: one login per person under a company account is a different design from a shared company login. Also decide who invites new users and who removes them. Roles and permissions covers the model in more detail.
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.