AFAIK the bike that you showed as "our" 660 is the 2005 bike and the "everyone elses" bike is the 2004 bike. KTM changed to the different rear fender and twin exhausts for the 2005 models.
As everyone else has said, I'd go for the Adventure from the start. Getting a 660 and then converting to Adv. spec. would take lots of time and money for little benefit.
A good friend will come and bail you out of jail...but a true friend will be sitting next to you saying........."Damn..We F*cked Up" - Anon.