OK, I can see this thread suffering the standard 'geekspread' and losing focus in discussions about things that aren't relevant, like rack systems, AC, power consumption and so on. Start a different thread if you want to talk about that, please.
I already have a case, motherboard, CPU, 16GB DDR4, 256GB SSD, many spinning hard drives. The downside is it only has a 1Gb ethernet connection and I5 4570. I don't pay for electricity, our solar easily covers far more than we use even in winter. We have a hefty house battery system that can nearly last us a week, so no worries for non-sunny days.
I'm wanting to get a server together so that we are not dependent on either of our main computers being active for either of us to access a shared pool of data. We'd both like to minimise the desk space for our main computers and have a box sat somewhere else that takes care of looking after the drives, reporting problems, maybe running some light network based QOL apps.
I'd like to saturate a 2.5Gb connection, and I have enough spinning drives to do that. I know seek times will be terrible, but I have a few SSD drives, so I could set up nearly a terabyte of cache. I know there are many filesystems and setups that can be used, ZFS, TrueNAS, and so on. Do any of these benefit from more than 4 cores? Are there benefits to using one over another? Are any of them free?