If I were setting up an audio only PC or iMac, which was my primary computer and I wanted it to be quiet, I would definitely go SSD for both OS and data. In fact I would probably use a 500GB SSD for the OS and applications, and a 2TB SSD for data, assuming 2TB was enough. Expensive, yes, but quiet, although not silent. (Why does an SSD make any sound at all?! All mine will make some noise when working hard.
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I have built a few "quiet" PCs now, and continually upgraded for silence, and still they just are not quiet enough for me, if there are spinning hard disks and fans, and they are near me, like my main PC in my office. If I needed more than 2TB of data space, my future PCs would use a remote located NAS.
Whether a Remote Control App on a tablet could wake a computer is quite dependent on the App, and your network environment, and if you want to do it from your LAN, or while you are outside your LAN and want to do it over the internet. Very possible with the right Router, App(s), Network Cards, etc. on a PC. Sometimes you might need to use a separate App to wake the MC Server, then start the Remote App. Some routers, and Internet Service Providers, do not allow WOL requests to be passed from outside your LAN to the MC Server. You would need to check that if you require it.
I would be very surprised if it was much different on an iMac. A quick search of the internet shows many people doing it.
My MC Server is my HTPC, and it wakes as required when I access it from my office PC, which is acting as a MC Client. Easy. Reliable.