Yeah,,,the 20 mm offset is critical,,,as for tires,,,it seems that 235/45/17 is right for your car,,,,,any wider,,,and the fronts can rub on the strut---- recently went thru a bad experience at a tire store,,where they wanted to mount a 'bmw' wheel that had 47mm offset, because they insisted it will fir a 5 series (47mm offset is for a 3 series),,,,well,,,it didnt fit, and in the process, it clipped my brake sensor wire (LF wheel),,,and i had a brake sensor alert come on. Make sure the offset is 20mm for all 4,,,and that your front tire width isnt over 235. You can go wider on the rears,,for a staggered fit, but you wont be able to rotate fronts for backs.
As for the price,,,what funds ($US, or?)
Look at what Bruno has to say about tire and wheels,,,,,
www.bmwe34.net
Art.