As I am sure you know, the M6 has been around for a while, the Summicrons even longer (more than 40 years in at least 3 iterations). It is easy to date cameras and lenses by their serial numbers (I and probably many other APUGers have lists). A relatively expensive but good shop in London has these prices:
As I am sure you are also aware, the Leica experience is also available under the Voigtländer name. There is also the question of whether you need the (considerable) weight of a 90 Summicron, whether you will ever shoot at f2 and whether this lens is capable of being focused totally accurately at full aperture. Another consideration is servicing - I know nothing about Leica service in Australia, if this is hard to find, you might like to buy a fully-serviced camera from a dealer - this will then keep going for a long time without attention.