I agree with some of this but not all. I'm not sure he will be a great signing but I think he will probably do well enough over the next two years.
You are right to point out his contrast to Templeton, McKay and Little. I think that this is why McCoist wants to sign him. He wants the option to play a big guy up front. I think that this is why he signed Kyle last year too. When he signed Kyle I was surprised as I thought McCulloch would play that role but I think we can all see that when McCulloch plays it will probably be at the back or in midfield.
You also mention his resale value. In an ideal world we would only sign a young player who fulfils his potential before we sell him for a nice profit. In reality the manager needs to balance that against the need to achieve short term goals. When you consider that alongside the constraints that we still face in the transfer market is it any wonder that we will make signings that are imperfect? I'm not sure that it is a big problem either.