The only add-on you need to have a complete MiSTer that runs all cores is the SDRAM board, since the IO board is only necessary for adding analog output (RGB/RGBS/RGBHV/VGA/Component to SCART/BNC/RCA/VGA/etc) and to house the fan. Using the IO board as a fan mount is complete overkill if you only need HDMI output though, and a cheap 3D printed fan mount will suffice. Based on the questions I’ve been receiving, I fear that a lot of people don’t realize this.
People keep asking if a particular core “needs” the IO board, when none of them do in actuality. The IO board’s secondary SD card slot is only currently used for dev use. Although the X68000 dev core technically requires the 2nd slot, this should be negated if/when the core is updated since it will likely move to the Virtual Hard Drive (VHD) setup that modern cores use.
The addition of VHD support in MiSTer has done away with the need for the second SD card and it’s possible that no cores will ever explicitly require the IO’s second SD card slot outside of dev use. The genius of using automatically booting VHDs within cores can’t be stressed enough, and is a testament to the creativity and ingenuity that Alexey has put into MiSTer. Support the Open-Source MiSTer Project here.