Recently volunteered (for a friend) to make a website for his business with same added features. I do have moderate web-dev experience but really wanted to try to use wordpress for this. Here are some of the features i am looking for, (will try to be as brief as possible).

1. Ability to create custom objects with custom fields (e.g. Orders). Then 
basically having some sort of ability to perform CRUD operations on the 
same. Field could also be of type image/images, where the user gets an option to upload picture for that field.
2. Ability to create fields on other entities of another type (e.g.
Order object will have a field of type Customer)
3. Expose APIs that allow us to change certain fields (for e.g. simple
/GET api which changes a boolean status) No requirement for
authentication and all that.
<http://www.website.com/api/changeorder?status=pending>

Would be great to get some feedback on above.