The wheels will fit with no problem, please see live example of the wheel set with the same size but more aggressive offsets et25 et30:
http://f10.5post.com/forums/showthread.php?t=1078346 And you can find more pics here on the forum, to feel sure about your choice.
Your dealer has recommended right tires, there are two perfect sets for 20": 245-285 and 255-295. Both sets have identical rolling diameters front vs. rear. My opinion your tires will sit like square or small stretch.
And you may want to consider some spacers 10mm all around. Good ones are from
Macht Schnell or
Renn Motorsport or whatever else if you want to.