Ferguson is quality. Does more than what people think. The season we finished top half, he was probably our best player. Hes the link between defense and midfield.
Only decent player? You're joking right? I doubt Guzan is any better then Foster? Guzans a decent keeper don't get me wrong, but Foster is better.
Your defense is shakey, and collins is a terrible centre back. If you're going to have a defender, at the least have them knowing the basics like clearing your lines, and oh..urhm tackling. thats what Johnson is, a good defender, not brilliant, but does the basics really well!
Fahey is extremely versatile, and McLeish really likes him. Adds Depth to your squad.
So, honestly mate, you could do with them all, maybe not Fahey, but almost certainly Dann, Johnson, Ferguson and Foster.