CRM Google App Script

I want a Google Script that can perform many CRM tasks that currently I have to perform using multiple spreadsheets. So, I want to have forms to enter and retrieve data from google spreadsheets; eventually be able to update some of the infos like, name, email, telephone, job etc without having to do it in the spreadsheet itself. I think on this App having 3 sections. One for registering a customer, the other that I can set date and time for meetings, and add notes about the meetings. The last section is for uploading a document for the meeting. The person who will have access to this last section must not have permission to edit the two previous sections.