I run a dual core mini-itx Supermicro Atom embedded mobo for the past 8 years. The only limiting factor is ethernet speed, even though it has dual Intel gigabit nics. I never have any problem and still do not. But my ISP upgraded my internet to 1G up and 1G down. The little atom CPU just cannot do 1G sustained transfer. 350m is about all it can handle.
I am pretty sure that supermicro server boards have 8 or 10 SATA onboard. And don't get a "consumer" add-on sata card, get an "enterprise" LSI (or OEM rebranded) card for $30 - $80. $30 gets you a Dell Perc H310 which can be flashed to JBOD mode and is highly recommended by guys runing Freenas.
SSD cache may or may not be needed. It depends. You need to think about your usage patterns. You can use something likes mergerfs that cache writes on ssd and then rsync data to hard drives after a couple days, or you can use bcache, or bcachefs which cache writes on ssd and sync data to hard drives, AND also sync data from hard drive to ssd for recent reads. And you can use both in the same system, with different mount points.