I recently took delivery of 2010 35d (I'm in Melbourne too) - and decided to stick with 18's for just that reason of taking to the snow etc (and I just wanted to get the best possible ride quality as X5's are a bit firmer than say your average sedan)... from all accounts chains for 20' wheels are really rare /risky / expensive - you really run the risk of bitiing into the wheels themselves - and they are very expensive should you cause too much damage - this is why the guys in the US typically carry 18's for winter and swap back to 20's for summer ..now that can be expensive for us in Aus as BMW rip us raw for parts etc ..but perhaps you could negotiate as part of your deal ? Other option is to go with 20's ...and just hire a 4WD when you want to snow it ...might depend how often you are planning to go up ...

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2010 35d Innovations
AW with great Tint

AW is the new black you know