Canon changed the button layout after the 30D. Look at the 40D, 50D, 60D and you'll see a totally different layout. The 450D and 550D are even more different -- specifically, there is no thumbwheel, no joystick button and all the buttons moved to the right side of the camera.
Also, the 450D and 550D feel very cheap (lighter weight, lower build quality) compared to the 7D and 5D. (I own a 450D and a 5D mark II).
So my advice is for you to consider a 7D -- maybe a pre-owned one if the price of a new one is too high. Also, a 5D mark I would be quite affordable now -- but going full frame will reduce the reach of your lenses (and your 18-55 won't mount on it).
