How is state shared between nunit tests?

There are some configuration settings that most of the tests in an integration test suite will need to share. These include database connection strings and similar items. It's very slow to fetch these from the system where they are stored. I'm planning to create a Fixture class with the SetUpFixture attribute that's in the root namespace of the assembly and use the OneTimeSetup attribute for the method that gets the config data. That will ensure it only runs one time before any of the tests start.

I can use a static property on the same Fixture class and then individual tests can read the config items with Fixture.ConfigSettings. This seems to work fine in some preliminary testing. Since the tests only read these settings there shouldn't be any cross test interference.

Is an arrangement like this a common way to handle this situation with NUnit? Are there other built in NUnit features or recommended pattern that may be helpful?

Yes, this will work. You should be clear, however that a SetUpFixture and a TestFixture serve different purposes. Do not use both attributes on the same class. Do not inherit one from the other.

As you noted, only static properties will work in this situation and the values should not change once set.