AI-Powered Invoice Processing Web Tool

I need a simple AI-powered web tool that automates invoice and expense processing for small businesses.
Here is what I want the tool to do:

Extract key data (date, supplier name, amount, invoice number) from PDF invoices using OCR.

Use AI (NLP) to understand and categorize the invoice (example: electricity bill, supplier purchase, etc.).

Store this information in Google Sheets (for testing) or later in a small PostgreSQL database.

Automatically send email reminders for unpaid invoices, based on due date.

(Optional) Connect to QuickBooks or Sage using their API (can be skipped for the first version).

A very basic dashboard/web interface to see incoming invoices and their status.

Tech Stack suggestions (you can propose better ones if needed):

Backend: Python (FastAPI or Flask) or Node.js

OCR: Tesseract or Google Vision API

AI/NLP: OpenAI API or similar

Frontend: Simple HTML/React or minimal UI

Storage: Google Sheets first, then PostgreSQL

Deadline: Within 3–4 weeks max
Goal: Build a working demo to test with small companies

Please include:

Estimated cost and timeline

Stack/tools you would use

Examples of similar work (if any)