I have read through the entire project description and I'm quite sure I've understood what you want. Firstly, I am a WordPress plugin developer and have developed dozens of plugins which serve a variety of businesses - I am amply qualified to build this. The one issue I see with this is that WordPress does not control emails, it (PHP) is configured to interact with another application, usually 'sendmail' to send emails and usually has no functionality to get emails. There is always a solution, but no one bidding can claim to they know how to implement this without knowing anything about the hosting environment (whether it is managed hosting (eg cPanel) or dedicated self-managed (eg AWS)). The most likely solution will be to use IMAP if the email server supports it. I would need to get that information before I could decided exactly how I would implement the solution on a technical level, on a high-level, it will just be a WordPress plugin. All of the other components of the system (Admin UI, inquiry form) are very simple. I hope we can discuss this project, thank you.