Some of the previous post are not completely accurate- but they are close enough to give you good inforamtion.
The models have evolved over the years- so you have to consider the year too. For instance, in 2004 an EXC model was an enduro bike for off road use only. In 2009 an EXC is a street-legal enduro bike from the factory. Other KTMs are easy to convert to street lagal bikes- XCWs come pre-wired for lights. But it takes a light kit and a lighting coil modification to accomplish.
Most of all the new KTMs have E-start (2T and 4T). Before 2008, many models did not.
You may be dissapointed with a 250 4T if you are going to ride much at highway speeds. But then again, it might suit you fine. For me, it is a little short on power.
Personally, I have a 450 XCW that I converted to a street legal. It is not so good on the highway- but it RIPS ofrf road.