not sure about 19 x 9 up front, usually with 19's on the clk, we go 19 x 8.5 front and 19 x 9.5 rear. you are looking at an offset of about +35mm for proper fitment. higher offsets will fit but you will need spacers to make it work. also i see you said the hres are 5x120, we use 5x112. additionally, hre wheels are custom made for each car and to the specifications of that car. after all that, i think its safe to say, they wont fit.
EDIT: also you asked about staggered being for looks or necessity. you dont have to go staggered if you dont want to. staggered does however give you a more aggressive, sporty look and with wider rubber in the back, gives you a bit more traction. clk320's come with 16 x 7 all around for example