Is it possible to link Commerce with an external datasource for products and catalogs? I know I can import using either code or CSV, but this is not a real option since product changes in the external datasource must be immediately visible in the website.
If you have an external ERP system then you could create an integration point between that system and EPiServer Commerce. You would maintain your products within the external system and push changes into a custom connector for EPiServer Commerce. You can make use of the WCF integration service or create custom code using the Commerce API to manage the creation / updating of products into EPiServer Commerce.
You may also be able to make use of an accelerator for doing this, through one of our add on partners, i-Core
It sounds like the product catalog is already being managed in a 3rd party system but if you are looking for more robust product management features you may also want to look at one of our commerce partners that have a great Product Information Management (PIM) platform.