Revo is spot on here. If you go with actual M6 wheels (in M6 offsets), you'll want to put spacers on the front (the rears will be perfect).As an FYI, there are two different "M6 Style" wheels. They are technically called "167 Style" wheels. M5 wheels are 166.
M5 and M6's use slightly different offsets. However, BMW makes an OEM 167 Style wheel in M5 offsets.
The 167 (M6) Style wheels in M5 offsets are rare in the US and can typically only be had from the dealer. And you're looking at around 7K for the set.
It's possible to find actual M6 wheels (in M6 offset - they will fit the M5 fine) used for what I see is usually 3-5K depending on condition and if they include tires.