Now chances are pretty good you could just find another tank rather than replacing both it and the bowl. I'm also pretty certain that you could go to most any hardware store and find universal gizzards that will fit almost any toilet. But one thing to keep in mind is the fact that new setups use way, way, less water per flush. Heck you can even get setups that have a button that you press once or twice depending on whether you went 1 or 2 which saves even more water.

As for a new rig not being tall enough to meet your wallpaper look through the moldings and find yourself a thin white one to fill in the gap. Plastic if you can find it so you don't have to paint it. Or you could even get a roll of that wallpaper edging and run a strip across the toilet and sink so it looks like you wanted it that way.