Unity is a cross-platform 3D game development tool for Windows, PC and Mac, and is available in both free and paid-for flavours. Adventure Creator is an extension (an "Asset") of Unity, that tailors it toward making 3D adventure games.
Licenses to download and make use of Adventure Creator cost $70 + VAT on Unity's Asset Store. Updates are free, but as Adventure Creator is listed on the store as an "Editor Extension", one license is required per seat - see the Asset Store Provider Agreement.
Important note: The one-license-per-seat rule was introduced with v1.53 on 1 August 2016. This rule only applies to those who purchased their first license after this date.
It depends on the game you're making, but provided it's a "traditional" adventure so far as gameplay goes, it's entirely possible to develop it without coding. Because of the modular design of Adventure Creator, gameplay and interface are totally separate. You can design and implement your gameplay with the default menus and GUI, and then go about changing it to your liking afterwards.
Not at all. Adventure Creator is designed to assist the designer, not to take control away. You can call your own functions from the event system, plug in your own events, or even disable select systems completely. Plus, the source code is clean and efficient - perfect for someone who wants to extend it's core functionality.
When it comes to VR, AC takes a generic approach that gives control to the developer to use it in the way they wish, rather than supporting any one VR platform in a specific way. The Camera system can be made to work with VR (see Section 4.8 of the Manual) - however some additional coding will likely be required to get things working the way you want. This should not be difficult for any coder already experienced in working with VR, however.