What should a supplier's site show before I send an enquiry?

Enough to place them, verify them, and know how to buy. Those are three distinct sets of information, and most sites do the first one and stop.

To place them: what kind of supplier, which customers and applications, which products, and how they differ from a trading company or a platform seller. To verify them: whether the company and factory are real, which tests and certifications the product has passed for your market, and whether delivery, installation and after-sales leave any evidence you can check. To buy: who to contact, whether a sample or demonstration is available, how quoting works, and what the minimum order, lead time and warranty terms are.

If the third set is missing, the enquiry you are about to send is doing work the site should have done. That is not fatal, but it tells you something about how the enquiry will be handled after it arrives.