Is Drupal Commons right for me? I'm reading a lot, searching, asking questions. Measuring thrice before cutting once.

Now it seems to be a straight choice between a pre-built Drupal Commons and roll-your-own based on core-Drupal plus some modules.
I absolutely require a wiki, which doesn’t seem to be in the standard distro, no easily addable as a single module.

I won’t mention standard stuff like forums, but will say that I want a “social networking site” for ex-pat foreign workers in another country.

It would be nice to have some form of Goups of fiends &amp; colleagues and to provide sharing permissions at that level of granularity (e.g, world read, group right/ group read only, etc); joint calendars,  access to blogs, maybe chat rooms or even one form per Group; maybe a photo album, although I suppose they can just refer to their Flikr.

Maybe profiles with some extra stuff like IM, phone #, etc,  where the user can control who gets to read it.

(Plus anything more that you can suggest for such a community, of course)

It looks to me like I would be best to start with Drupal Commons – do you agree?

The only thing is that when I go to download it, it says 
6.x-2.0 tar.gz (17.4 MB) 2011-Aug-26 Version 2.0 is a major upgrade
Which seems to imply that it is based on Drupal 6, not Drupal 7. Is that correct? Is it a show-stopper for me? Or can I just install it and upgrade everything?

Sorry if I don’t sound too knowledgeable; I am still not a Drupal user, just a dabbler on my local PC.

Thanks in advance for any help.
I would recommend starting with Drupal Commons as a base. It comes with a lot of prebuilt configuration that's very nice, like the fb-style walls for groups and for users. You get all the standard things you mentioned except for forums, but that comes with Drupal core, or you could go for a more advanced solution. You can disable any features you don't want in a per group basis. Commons already comes with a nodes as profiles solutions, so you just need to add the fields you want to the profile node. For chats you will also need an additional module, there are several, maybe this will help get started.

To answer your question more directly, yes Drupal Commons is right for you. It will save you a lot of configuration time, and you can see some best practices in configuration from what they did. Remember to back up your db often and don't be afraid to modify everything to suit your needs, you will learn a lot that way. As long as you have a a backup you can always revert.