Recomendations for where to assign the hash (#) key
I assume you're on a non-US keyboard - my US mac keyboard has shift-3 as #, where my £ is on my UK keyboard.

I think this rather depends on your key usage. Most normal users never use the ` or ~, so that top left key would work. I use them both quite a lot though.

Another alternative would be to use System Prefs to switch to the US layout. Most of the keys are exactly the same, and you'd get the usual shift-3 for #. Not quite one-button, but an easy solution, nonetheless.

F5 and F6 doesn't get much use on my keyboard at all, nor do F13-F19.