@george_ It depends on the UI framework whatever phone you're going to use uses, and that you can test/develop against on your desktop/whatever.
@george_ Well, whatever your current environment is, is good enough starting out with to play with KDE Plasma or the GNOME equivalent or what-have-you. I mean, if you're looking for a project you can build a something from SBC+TS. But if your goal is "I want to play with app development" you don't need new hardware. There's many ways to give yourself a virtual machine or just run it on your desktop.
A nice little Mastodon instance. Mild trolling encouraged (keep it local), but not required. Malicious behaviour is not tolerated. Follow Wheaton's law and you'll be fine.