As far as I know, the Odoo follow the modularity and that is the reason behind become very very strong into the current market 

My personal advice on this to understand the client's need in a very profound way then try to cover all the requirements with existing Odoo flow. Explain to them why what the currently has in a real case.

So that you should think about the real client business flow and cover all the scenarios with default flow of odoo and add very very fewer fields and its field structure details into your new module.

As per my perspective 

1) Is this advisable?
- As per the client's need and requirement of existing client business flow.

2) what would happen when updates are given in the module? ,
- If the client gives you a new update or want some more additional functionality then you should create a new custom module based on the new business flow and add the dependency with base modules of Odoo.

3) Is it better to start a new module?
New business flow has more and more complex needs a custom module to solve the current problem of the client. The new module helps you to arrange the proper structure and maintain a very long time. 

4) Is it possible to copy the existing modules to have them as a basis?
-Not good practice to copy the existing module but better to add the dependency of all other existing modules as per the client business flow

4a) is it recommended?
- Not recommended for the same

I always use the Studio App to make changes. Then the changes is automatic protected during upgrade. 

But - if you do many changes you will have to test them all before go in production with upgraded system. 

But I would definitely recommend the Studio. 

Hi Johnny:

As long as you follow the guidelines recommended by Odoo for extension and customization, there are usually no major issues during updates within a release, for example, daily releases within Rel 12.

When you upgrade from one release to another, for example from Rel 12 to 13, you should do exhaustive testing of all your customizations since there are many design changes that get implemented in the core product.

In both cases, regression testing is highly recommended to avoid unwanted surprises. Having said that, I have found the layered design strategy adopted by Odoo to be highly flexible and stable compared to other enterprise level products I have worked with over the last 2 decades.

Hope this helps.