Just built one myself. Here’s what I used:

Jonesbo N2 case Topton/CWWK/BKHD NAS motherboard from AliExpress - includes N5105 or J6412 processors, 2 M.2 slots and 2x ddr4 slots. They haven’t started rolling out N100 boards yet otherwise I would have gone for that. 32gb ddr4 EVGA supernova 450gm power supply. Overkill but they don’t make many sfx power supplies that are fully modular. 2x1tb nvme drives Case has 5 hdd bays for a pretty decent amount of media storage. Cost me less than $600 USD not including any HDD. Modifying this a little could probably put you under $500

Prior to this I used an old Optiplex 3040 sff ($100) with a 1tb HDD and core i3 6100 and replaced the cd drive with a SSD bay so I could boot the OS off of that. It worked for a budget but didn’t have any expandable storage.