The fact of the matter is that the wikia article you saw is wrong. The NISSAN Skyline GT-R V-spec 1999 is not available for purchase in the Offroad class. It's only available in Drag, Drift and Race.
In a different page of the Wikia you can find the classes the car is made available in. The 6 icons on the top represent which classes the car is available in, the third is Offroad and it's grayed-out, not available.
You were, understandably, confused as the 1999 GT-R page of the Wikia (used for your screenshot) displays that it is available in the Offroad class via the third icon is filled-in when it should not be.
In fact, there price of the car depicted in your screenshot ($88,700) is actually the value of the 1993 NISSAN Skyline which is available for Offroad purchase! Confusing, right?
In short, the NFS wikia is inaccurate and you can't buy it. Sorry.