Price and stock monitoring script for suppliers

We are looking for a developer to build a script that automatically monitors prices and stock availability across our suppliers. Right now our purchasing team checks dozens of price lists and websites by hand, spends hours every day on it, and still misses important changes. We want a system that regularly gathers data from a defined set of sources, compares each value against the others and against previous readings, and alerts us the moment a price goes up or down, an item goes out of stock, or something becomes available again. The sources vary: some suppliers provide exports in CSV and XML, some publish prices only on their web pages, and a few keep everything behind a login-protected account area. The script must handle all of these on a schedule, deal gracefully with failures, and keep running even when one source is temporarily unreachable. History is essential: for every product we want to see how the price and stock level changed over the past weeks and months, with the option to export it to a spreadsheet. Notifications should go to email and to a messenger, with flexible thresholds, for example reacting only when a value shifts by more than five percent. A simple panel where a manager can add new sources and products without a programmer would be a strong plus. Please tell us which stack you would use, how you would keep the collection stable, and your realistic timeline.