It goes without saying that you better calculate your needs first, and then decide accordingly.
In my opinion, Vitz 2006 has a bit unreasonably high price tag.
About parts availability, so far, Vitz 1999-2004 are better placed as most of the owners suggest. Nevertheless, 2005-onward might also be catching up quickly i suppose. The owners might be able to show a better real picture.
About city, yes, Karachi is perhaps the best, if you can manage going there. And in case you decide so, it would be better if you call Altamash (@alta) for further help (though he is hard to find on his cell phone). His repute, as a dealer, has been very good on PW.