Bushes will give you a problem when going over humps etc, not at highway speed. Your car would of course be going to one side if that was the case or be all over the road, and you not complaining of this. So dont think it is bushes.
You say the vibration is only above 100km/h then it must be the wheels itself, what happens the moment you touch your brakes, do the vibration increase, i.e get worse or not. What you also can do is jack the car up, wheel at a time and then spin the wheel freehand. Listen to the wheel to hear if there is any screeching sound like a grrr - grr -grr (have no idea how to explain this in english). If your wheel does this then it is the disc's that needs skimming or new disc, but do this first before going to put new wheels on the car. And do all 4 wheels.
Hope this gives you some ideas to look into the problem.