Coming from Navision 4 to Dynamics 365 Business Central will inevitably be a big change. There's been so many changes from your version of Navision, that while the core terminology and functions may share similarities, it'd be worth having some in depth demos of what it can do.
You may also benefit from some time with a Dynamics partner to talk you through what an implementation would look like, and they may also be able to assist with some scoping or business analysis.
Feature/function wise, Business Central could definitely handle manufacturing in SaaS, but whether this is right for your business is worth talking through with a partner :)
