It really depends on what you're trying to do. At the end of the day, the foundational components are pretty standard across the board. All machines have a CPU, motherboard, storage mechanism, etc. Oftentimes those actual servers have a form factor better suited for rack mounting. They often have more powerful components.

But at the end of the day, the difference isn't as striking as most people not aware of this stuff think.

I'd say considering this is your first experience, you should start with converting an old PC due to the lower price point, and then expand as needed. You'll learn a lot and get a lot of experience from starting there.