The data would need to be shown when a node is displayed - I would like the data to be accessible from within views, but also searchable, e.g. display products where stock level < 3 for supplier xyz.
How can you say I want the data to be integrated into my website -- but not imported into Drupal. Your goal sounds like alot of work.
You could try something like Remote Entity. But honestly I would take some time to learn the Migrate module and use Rabbit Hole to insert the data as "hidden" data to drupal entities or content types in my site. Migrate is super powerful for this type of syncing of data (syncing being a topic I think you're glossing over).