Not quite a completely in-depth guide, as some of the classes that you said had no specials actually do (Sniper can get headshots, Pyro can reveal Spies, for example).

But, I'm now MC's resident Engineer, and I found out that a dispenser, when located next to a Sentry gun, gives you a near-limitless supply of metal. Except when those pesky spies sap your Sentry, which knocks out your dispenser, too. The solution would be to have your teammates place multiple Sentry guns farther away, but still in range of each other, so that if a Spy knocks out one of them, he reveals himself, and then all the Sentries can open fire. The trick is to keep all the guns alive, and multiple Engineers in one area drain all the metal around quickly. I've lost many a loyal Sentry to too many Engies in one place.