This investment company i'm supposed to implement odoo  needs to handle mostly  debtor related issues like their payments in instalment  (so for each instalment payment receipt has to be given to debtor)  and with different interest rates for different loans , patently for late payments and employee bonuses for debt collection 

and investments and their interest in time

I'm new to the odoo so please can i know these use cases can be fulfilled with existing modules or do i need to create a new module? if can what changes can be made to the default flow ? (can the sales module be edited?)

And what version of odoo shall i choose 8 or 9??