Customize Navision by end-user

Hello all,

I'm French hope you will understand my English.
We plan to customize Navision for our needs.
We have two Navision 2015 , one French and one Spanish and as the accounting is  Anglo-Saxon we need to customize the application to feel like a French/Spanish accounting.
We need specific report and function.
We start with local French and Spanish partner but the cost per day between 500/1000€ and every modification take days, make us think to do this internaly
At the same time the UK compagny is customizing Nav to fit our Business and we will in futur have this application.

Seems then that we need  granule(7200) to modify Navision

Do we need a Granule per Navision or just one and  order object in the Navision production server.

I saw that Granule 7110,7120,7130 are prerequisite for Granule 7200,  we need to buy these ? for all the Navision ?
What will be aproximatly the cost of all the granule needs to do internal dev.

I'm a Powerbuilder/MSSQL dev since 17 years and just need one developer license of powerbuilder for all the project.
Seem not to be like that in Navision