Ben's answer is excellent, and should be marked as correct. This is simply some additional information to reinforce the decision to use Craft 2.
The current version of Craft 3 is a
developer preview beta (thanks Simon)... This means that there are no implicit claims of stability, and anything could change at any time. Similarly, there is no guarantee that you will be provided with a clean migration path for your data, if something were to change internally.
As you noted, Commerce is not yet compatible with Craft 3. It's possible that Commerce won't be adapted to C3 until the official launch of C3 (tentatively slated for late 2017).
Finally, upgrading your completed site from Craft 2 to Craft 3 should be (reasonably) painless. The facets of the site which do not directly depend on plugins should be a nearly seamless transition to C3. For the parts of your site which do depend on plugins, it will mostly be a matter of adapting those plugins to the new C3 architecture. While this will probably amount to a small-to-medium sized challenge, it will require the exact same learning curve as you'd be taking on if you were to jump into Craft 3 now. In fact, the learning curve would (theoretically) be easier if you waited, since the Craft build would be considered stable, and there may even be some documentation (and/or tutorials) available at that time.
In summary, definitely go with Craft 2 for now.