If you feel most familiar with Debian, I recommend looking at Mint. It has great compatibility and is otherwise easy to use, and it handles GPUs well (including Nvidia). In terms of hardware, I’ll agree with the others here–used is the way to go, you’ll get a lot more for your money than buying new.

That being said, you can also look for clearance and/or refurbished PCs. This one would probably be powerful enough.

Doubtful. Without Linux running the majority of the internet, I doubt the desktop scene would look the same as it does today.