Hi @Egen, welcome to the forum 
Cool! I’m excited to see how this goes, if you’re willing to share your progress 
In theory ArduSub should work on most ArduPilot-compatible flight controller boards, but the only ones we’ve done any significant testing on are the original Pixhawk, and our Navigator.
For reference, there is some existing advice about the Pixhawk 2.4.8, and there have been some discussions about the Cube Orange.
No, there’s nothing special about the Raspberry Pis we sell, and it should be fine to buy one from a local store.
If you’re planning to run performance-heavy extensions then you might want to get one with more RAM than the base 2GB model, and if you’re not buying a pre-flashed microSD card from us then you’ll need to make sure you get one with a high quality (we recommend Class 10) and a decent amount of storage (we currently suggest 128 GB, although we were providing 16 GB by default until quite recently, which should be sufficient for standard use-cases).
BlueOS can be installed on Raspberry Pi 3B and later (e.g. 3B, 3B+, 4B, and we’re working on Raspberry Pi 5 support) - the Raspberry Pi 4B requirement only applies if you’re using it with a Navigator flight controller (which we’re also working on enabling Raspberry Pi 5 support for).
If you’re familiar with Docker you can even test BlueOS on a more standard Linux/Mac computer (with some reduced functionality, to avoid causing issues with your computer’s hardware and network access).