I agree with this...
Buffalo courted Cowher for weeks and he had no interest, but recommended Gailey. Shanahan had what..? An 8-10 hour interview with the Bills and said no thanks. Gruden would stay in the booth before he takes on a massive challenge like Buffalo.
The only real shot the Bills organization has at hitting a home run with a head coach is going after a top NFL assistant or coordinator or possibly a good college HC. Though, as /dev/null says, they'd have to actually accept the job which is no guarantee.
In my opinion, the best way is to take a chance on an NFL assistant... and hope they accept the offer. Buffalo had this chance in the past with guys like John Fox, Marvin Lewis, Leslie Frazier, etc. But of course, they didn't end up here... What a surprise.