Even my 1 VDEV Z3 pool manages playing 4k just fine over a 1GbE network connection.
I agree that buying used hardware, especially quality cases, is the way to go. There are many good mini-itx motherboards out there, and as long as you know how to spot potentially problematic ones (ie c2xxx series embedded processors with the potential avr54 bug) it’s the older motherboards that offer the best bang for the buck AND adequate # of SATA ports.
There is a reason that Supermicro still makes x10-era motherboards today and that’s all the customers who still want boards with plenty of SATA ports. More modern iterations of their motherboards simply eschew them in favor of NVME (which may be the future but also wicked expensive $/TB ATM).
If the OP goes the AMD route, I suggest looking into the Ryzen Pro CPUs with the G and GE ending which are available used and offer much lower power consumption at idle than regular Ryzen CPUs. This is particularly interesting in the context of SOHO use where long times at idle with little power consumption is pretty welcome in places with high power prices like mine.