Fixing water ingress at mitred corner in roof fascia
Metal facia can look awesome.  

For the amount of work that you are proposing you should consider a 3 part system.  A top piece with a hem hook that looks like an h that caps your roof felt.  The roof felt and top face of facia inside the bottom of the h.  A bottom piece similar idea but flipped on the x axis.  The last piece, a flat piece of metal with a female hem along each length that hooks onto the two male pieces.