Hi,
There is no straightforward answer to this. It all depends on the need and on your plan.
Dynamics 365 Sales, Customer Services, and F&O are capable of running independently, however once integrate, you can best out of these.
Again, I'm not aware of your project, however, as per my experience, in most of the implementations we started in parallel.
So we plan out phases, In Phase 1, we implement Sales in parallel to F&O. This means anything, where you need integration with F&O will be done either in Sales or outside the system. For instance, normally, the sales process until the Sales Order stage is managed in D365 Sales, however, invoices and fulfillment happens through F&O. Since D365 Sales do have capabilities to prepare Invoices, so in Phase 1 your invoicing can be managed in D365 Sales or Legacy systems. In Phase 2, you can start integrating D365 Sales with F&O. For example, now once a Sales Order is created, it flows to F&O for fulfillment and billing purposes.
One more example, there are processes where there is no direct integration with F&O. For instance, our Marketing Department mostly cares about Customer Segments, Campaigns, and Customer Insights. None of these are directly involved with F&O. So we can safely, start with Marketing Module Implementation.
Similarly, you will find scenarios in your project.
I hope, it gives you some perspective.
Feel free to discuss this.
Best,
Wahaj
