Here's what I gather so far...
Coin doors come with the slot where the coin goes in, and the trip wire that triggers the switch, and a place where if the coins fall there they come out the return. But there's nothing in between the in-slot and the trip wire unless there's a coin mech installed. The coin mech chooses to accept or reject a coin, and guides it to the trip-wire. The brackets for mounting coin mechs are pretty standard so most mechs fit most doors.
Whether the door comes with the mechs or not depends... Happ ships their coin doors with quarter acceptor mechs, and will swap out for the mech of your choice if you call them and ask. If the guy on eBay doesn't say the mechs are in there, better assume they're not until he tells you differently.
I don't know about mounting hardware though, the coin doors get mounted in a variety of materials so you probably have to go buy the appropriate screws / bolts / duct tape for your cab. That probably also depends a lot on who you're buying from.
Oh yeah and there are little transparent inserts to make the lighted coin-reject buttons display different things, like "Insert 25 cents" or "Insert Tokens".
My 4-player door will have custom inserts that say 'M' 'A' 'M' 'E' on 'em.
