How to start developing and debugging the software part for a quadrocopter?

Question

Quadcopter

Good time!
Started developing for quadcopters. I am interested in writing and debugging my own algorithms for the operation of propellers and processing the available data from sensors.
I don't know where to start, what to buy... There are a lot of Quads. There are also a lot of various remotes, etc. But I found only DIY Drones with open source.
Is there such a solution like a quadric dev kit so that you can build and code for it as much as you want?
Thanks to all!