Unless you’re hardcore I’d highly suggest not getting an actual server, especially a 1U server like that. Servers are loud, use a lot of power, and especially in 1U form not that expandable. CPU and RAM upgrades are cheap, but say you want more drives, or to install some weird expansion card it might not have the space.
If photoprism is actually “AI” you’ll want a GPU to do the processing, and 1U servers limit you to oddly sized 1U GPUs. But considering they say it will run on a raspberry pi I’d assume any desktop with a core i7 would do the job. If you can find a desktop with 4 ram slots of DDR5 that would get you plenty of expandability. The DDR5 spec is rated for up to 512gb per stick, so assuming the memory controller (and bios) supports it you’d be PLENTY future proof. But even DDR4 with 32gb sticks should be plenty, and those machines are CHEAP.