Hi Oliver,
yes, you can access a data source in a business rule. For this you can select the data source from the Objects tab and then filter it / compare the result.
I had a similar case:
I needed to upload a new version of the document but it was necessary to delete the old document to prevent access to old versions.
After uploading the document, I stored the item id in a technical field.
When a new version was uploaded, it's checked via REST, whether the item still exists.
If this is the case, the item get's deleted.
Since the REST action throws an error, when the item id doesn't exist, I stored the response in a local parameter of the automation. It's necessary to switch "Continue execution if http.."
Afterwards I checked the ResponseCode in an if operator.
Best regards,
Daniel