Dou you want a full A4 landscape per page or half A4 landsacape per page (with a fold in half A4)?

So if you rotate a A4 paper 90-degrees, that should be a good enough height

Dear Project Client,

May I know what is the Handbook document brochure format you need for this project? Can it be created in Indesign and convert to PDF format for your future use?

Apple

Yes, InDesign would be most ideal. For the printers, they would just need it exported into PDF format, but as you mentioned it allows the Flexibility of updating the content in future.