My company is going to use odoo v8 to manage the business, and I am responsible for the project.

Since I am new to odoo/openerp  and lack of experience of it, i am not sure what is the good practice of developing with odoo.

My company has business of sales order, but the business logic is almost different with which in odoo originally. All products of us are services, and the list price is strict. Moreover, the workflow of SO is totally different. 

Should I extend sale module, inheriting and modifying the existing one, or just make a new module in such case? Kindly give me some hint.