Billing an Due Dates

[dym]

Hey guys!
Im trying to migrate to WHMCS but im confused regarding the billing options.

I have pre-existing clients that have individual products/items that are on recurring billing.
These items should be billed on every 1. of the month. How can i accomplish this?
Would this be a case for "billable items", or adding a hidden product? 
In most cases the products vary, so no two customers have the exact same product.

But the most important part is the billing timing. I want the invoice to be sent on the first, for all pre-existing customers.
How would that work? 

Greetings, 
dym