I need a CRM and invoicing system in Google Sheets

I'd like to create a CRM and invoicing system

• Needs to be in Google Sheets, as I don't have Excel and don't want to get it

Each customer has a package of services and prices. I need to be able to assign 1 or more sales items to each customer, with the ability to check each item off. Each item should have a deadline, with options for several reminders on different dates.

When items are checked off, I should be able to issue invoices

• Auto generated sequential invoice numbers
• Set text and logo on each invoice to include my company name, bank details etc
• For each client, I want to be able to define whether I output invoices and emails to them in English or German. Text will be provided

Customer details taken from a separate sheet will be entered onto the invoice.

• Each customer can have up to 5 address lines plus VAT number
• Each customer can have up to 5 free floating lines, as shown in the attachment below. One of these lines may be used as a summary in case a statement is sent
• Each customer has a target payment date (either 0, 14 or 30 days), which is defined in the database sheet or per invoice
• Product details taken from a database will include product name and VAT amount, or can be entered freely
• Can amend tax rates and add a discount if necessary – either per item or overall
• Tax rate and discount column is hidden on outputted invoice if all columns are identical; instead, this number appears only at the bottom
• Auto calculate totals

When finishing an invoice, running a workflow or pressing a button will save the invoice to a specific folder on my computer (in dropbox) and also set up an email with auto subject and body

• Invoice template will be provided, and should have ability to fit onto one side of A4, or continue on second sheet with a note showing page number and totals at the bottom
• Ability to mark an invoice as paid
• Easy filtering for unpaid/overdue invoices
• Ability to send a statement, showing unpaid and overdue invoices. Template will be provided

Interface must be user friendly and easy to follow. Changes and updates may made later, at a fixed or hourly rate