My advice (I know it's not my money) is to buy a good quality motherboard, good quality case, good quality power supply. These three items will likely last you well over 10 years. The case I purchased for this project is made of nice thick steel and will last a very long time. The downside is it's heavy to move around, especially when it's loaded with hard drives but it has outstanding air flow properties. I bought it on sale too so that didn't hurt at all. A good quality power supply will last a long time too, jsut keep an eye on the fan and if it's not rotating properly, replace the fan with a high-end fan, this really can extend the life of that power supply. Oversized fans in power supplies mean a quiet power supply and better air flow. A good quality motherboard with high quality capacitors (the component which typically fails first) shoudl keep you going for well over 10 years. Honestly you will likely replace the motherboard due to it just being too slow for the technology in 10 years or more likely because we like to upgrade our toys for no reason.
An ITX motherboard is fine if you need ultra small but if you plan to stuff a lot of hard drives in the box then why by a small motherboard. My sincere advice is to figure out how much storage you need, how many hard drives you plan to use now and plan for some future expansion. Now you can choose your case and then select a motherboard to fit into it.
If your goal is to have your NAS sitting in your bedroom at night and not keep you awake, you are on an uphill battle unless you just power it off. While you can make a very quiet system, in the dead of night even the smallest of noises will seem very loud. My system is very quiet but my wife tells me I have super hearing at night but during the daytime she can speak right at me and I can't hear a thing (selective hearing).
You said that you will make a posting in "Will it FreeNAS" and just include all your goals for the system, be as specific as you can and we will give you sound advice.