FTDI drivers can be downloaded from the FTDI Website.
You should then be able to install the driver using "Update Driver -> Browse my computer for driver software" and selecting the folder containing the extracted files.
This is very similar to a project I had in mind, also using an SDR for radio spectrum mapping.
Unfortunately there is no ready-made solution to your exact problem, and specifically there are no entirely Pi-based drones that I know of. But we can get pretty close by releasing the constraints a bit.
The simplest build would be an autopilot-equipped copter with ...
Yes. This is how flight controllers talk to the already available sensors on board*. The addition of barometer and magnetometer sensors (and any other sensors that these flight control softwares support natively) is, and should be, done via the i2c bus. Save some sensors like GPS which is generally UART (though i2c/SPI GPS chips may exist).
You might check out this video on using a Pixhawk and a rPi with Python. The video just walks you through the serial connection and some Python code for basic control. The Pixhawk is doing all the real time control (Linux isn't good for that part) and the rPi is talking to it over a serial port.
You might also look at the MicroHawk reference in: Do drones ...