should be no major issue buying the Blackline alloys. but they will be expensive i imagine.
whenever changing wheel size, you also need to change tyre size (profile) to ensure the overall circumference remains virtually the same. a 3% difference is acceptable, preferably towards the 'bigger than usual' side; this is cos the factory speedo usually 'overreads' your actual speed (ie: indicated speed > actual speed; eg: 62mph on the speedo = 59mph in reality). so a bigger tyre will make the speedo more accurate, which i find preferable.
i can't tell you what tyre you need cos i don't know what the original tyre size is. though if it's on a GT, then 225/40 for the 18" is the usual size (just unsure if you're planning on using them on your 156).