A) HP Mini second hand. Low power in the "T" CPU models, some have 2 nvme slots that can be used for extra storage with a cheap adapter like this + a power supply for the hard drives. If you don't want to DIY it so much some also have USB type C ports (and Thunderbolt) that you can use to connect to an external drive enclosure or this one.
B) SSD for boot drive, run VMs etc, HDDs for long term storage
C) Debian as base system, no GUI. LXD/LXC as hypervisor to run all your stuff in containers and VMs. Or run everything directly on the machine.
Other recommendations:
Use BTRFS as filesystem as much as possible;
Aside from the big brands like HP and Dell there are other alternatives such as the trendy MINISFORUM however their BIOS comes out of the factory with weird bugs and the hardware isn’t as reliable - missing ESD protection on USB in some models and whatnot;