Hi Pascal.
Basically, from Sfin point of view, customazation doesn't require a lot off efforts (and costs)
And it's periodical procedure(let's say like archiving), so you can run it when you want.
But there are some bottlenecks - access to aged data. It could have high price for project(especially if you have a lot of customer programs from old systems like ERP6.0 )
That's why I strongly recommend read note 2170722 - Overview: SAP Data Aging for SAP Simple Finance (in this note there are some reference notes which also very useful)
So you, PM or smb else, should decide - include it in the 'main' project, or shift it for future, but anyway you need invest time and money.
According to me, you need at least 2 person - 1st HANA/BASIS expert, 2d SFin/Fin/Abap expert who can dig deeply