Pulling in addl PDFs for variable document length

Just starting a project. I have an InDesign file with 8.5 x 11 pages which will compose into booklet form.

I have a list of customers and their customer number.

I have another list with a series of customer numbers relating to a particular two page PDF file name to be inserted into the end of each booklet. Some of these customers will get up to three of these two page PDF files. So in addition to adding multiple PDF files with the same customer number, I also need to insert 2 watermark pages as filler when the quantity is not divisible by 4.

So my list pertaining to the PDF files may look like this:

Customer # 11911 - inserts PDF file name St. Marys

Customer # 11911 - insetts PDF file name St. Johns

Customer # 11911 - inserts PDF file name St. Albans

Customer # 11912 - inserts PDF file name Mr Roberts

Customer # 11912 - inserts PDF file name St. Albans

Customer # 11913 - inserts PDF file name St. Marys

The covers are being printed on press, imaged with customer name and address and then inserted as the b&w inside pages are imaged.

So right off we know these booklets will be different sizes and different weights, so first thing we we will probably need to do is to separate the data based on weight, correct?

Can someone now tell me how we may proceed from here? We want to make this as simple as poosible. I believe the mailing is right around 1500, so it’s not real huge. I also do not know how to write code.

Thanks