Are they really more muck-resistant than geared setups (getting tired of chainsuck & shifting problems riding in Pennine muck)?
Yes, that's pretty much the only reason I have one.
How easy is getting the chainline sorted?
If you're not silly enough to use a SS specific hub and just use a normal back wheel with sprocket/spacers then it's very easy.
What sort of ratio would be good (bearing in mind I don't want to kill my knees)?
Start off around 2:1 – 32/17 for me – find a gear that works on your geared bike and do that.
Is facial hair also a requirement for riding a bike with one gear?
No, luckily as I look stupid if/when I grow a beard.