I am trying to implement some middleware that notifies an external service of changes to certain entity fields (Vendor fields, for example) in NetSuite. Is this an appropriate use case for SuiteScripts ? It has to have the ability to secure login tokens/credentials to authenticate and post against some external endpoint.

SuiteScript / Suiteflow, you can use both, Probably an after submit (in some cases before submit) should make do this part.