As we grow the business in the States, do we have US tax liability even if we don't have a physical presence?
Possible but unlikely; talk to an accountant. It will depend on a variety of facts which are difficult to assess from your post.
Do we have to collect US sales tax?
If you have a nexus (physical connection) in a US state, you may be required to collect and remit sales tax. Historically, it would be pretty easy to judge that you don't have a nexus. However, states are starting to become more expansive in their definition of what would create one. You may need to talk to an accountant.
Opening a warehouse in the US will almost certainly result in you having a nexus in the US state in which you open the warehouse, which will result in an obligation to collect and remit taxes for that state.