I'm ready to graduate from my Raspberry Pi era of selfhosting and buy hardware specifically for use as a server.

I've been recommended in the past to look for used Lenovo Thinkstations and/or Dell Optiplex, but it has been so many years since I've shopped for a computer, I don't know what kind of specs to look for. What are the types of specs I should look for to get the best value for money?

I'm hoping to spend around $300-400, get something that can be upgraded in the future to last 10+ years, and do the following things:

• YUNoHost / reverse proxy
• Nextcloud with a custom domain for email addresses, cloud drive, photos
• Music Streaming with something like Navidrome
• Serve static websites
• pi-Hole
• Maybe pi-VPN

And someday maybe:

• Host game servers like minecraft
• Jellyfin for videos
• Kodi and output to TV?

So far based on my selfhosted journey, I expect to want the following:

• Room for 3+ Hard Drives
• External UPS (probably will go with the cheap APC at Microcenter that's always on sale).
• Solid Power Supply / Cooling
• probably 1000 gigabit Networking (?)

The types of questions I have for Thinkstations / Optiplex:

• How is the Power Supply / Cooling?
• Processor? Do I need i5? i7? Generations? AMD? Clock Speed? I'm completely lost here.
• How much RAM do I need?
• Do I need a discrete graphics card? Can Thinkstations / Optiplex have a graphics card added to them later?
• Anything else I'm missing?

Thanks!