I'd keep McAllister for the simple reason that he has something a bit different to the rest of the squad. He's hot and cold (definitely more cold recently) but I can't think of another player we have that, on his day, can beat players and carve open chances.
Erwin, Dennis, Main (although, as said, he's on a 2 year deal) can all leave. I'd bring back Jamieson from Inverness to replace them if we can't get anyone else in. McGrath could well go if the money is right.