Tips on Reliable Flight Data for Hardware Flight Tracker?

I know this question has been asked a bunch in various subreddits, but I wanted to see if folks had tips based on what I'm building. Basically, I want to build a desktop LCD screen that shows when a flight flies over you within a certain radius and then shows you basic information like the plane, it's route, etc. I'm an avgeek myself and plane spot at SFO a few hours a week. When I talked to friends about it, we had lots of ideas like creating alerts for squawk codes, plane types, etc. Disclaimer: This is a product I intend to sell. As a bunch of people have already complained, getting this da