First of, there is a nice set of documentation for V8 and further down the road:

https://www.odoo.com/documentation/8.0/reference/orm.html#inheritance-and-extension

This should at least get you started. If you want to developer it yourself, then the most logical steps in my humble opinion, right after create the basic module are:

1. Activate the "developer mode" in Odoo, to get more information while using/browsing.
2. Find the class for payment.
3. Inherit it and add the 2 boolean fields. 
4. Find the button name and look up in which module it is defined.
5. Inherit that class and view to add "attrs" to the button, stating that it should be readonly when either one of the booleans is unchecked.
6. Restart the server and install your module.

Hi, 

I have the same kind of issues and not able to figure out easely. 
I m talking about issues because a lot of basics are not available quickly or need developments. Among them : 
 coupons, 
newsletter option when creating an account, 
customer account functions as history view, return function,... 
automatic account creation while buying something
many pictures per product, 
function to add image per each variant value (now image is related to combination off all values for a product)
....

I m not a developer but have 15 years experience in IT and web. May be i m wrong and i m not enough smart to understand how to set functions in odoo ( i m on that since 2 months now), but i have the feeling that you can reach 80% of your needs (and unexpected great functions) really easely but for the rest (and i m talking about basics and minimum) it s  really hard or not possible without specifics... 

Now from my experience, and may be i m wrong, i feel that odoo is really easy to sell and  really difficult to implement for e-commerce and pos modern projects. 

Odoo is so big and powerfull, it should be great to find a way to release only ready to use module as for me e-commerce isn t ready.

Odoo = Amazing  job...but not complet, and how could it be ? web technologies/functions are so creative that if you cannot up to date your solution often you will soon be out of the game. So is only saas version possible ? and in that case when minimum functions will be available ? every 18 months ?