If the machine is hosted on consumer internet in your shed then you're likely to hit bandwidth issues before you really suffer much from CPU power. Just uploading files for users to download isn't very demanding on your CPU really, especially if it's mostly the same files so they're cached well.
Discussion
Thanks Pre, that’s the part I’m really unsure of. I currently have a decent plan with my ISP (although they often seem to fuck around in the early hours). I’ve been pleasantly surprised by the download figures since switching from Peertube, whilst that obviously doesn’t mean active listeners, Castopod seems to provide a better experience and I’ve yet to get DMs/messages about stuff not working, which was the case with Peertube.
The hardest issue now seems likely Yunohost issues that I can’t solve as most instructions involve straight CLI set ups.