I would go with the drz400. Its capable off road in stock form, with minor tweaks you can ride anything. Highway is no problem just gear it right. If you buy a z get a set of 14/47 gears for off road use. This alone will drastically change the bike, then switch out to highway gearing for your commuting. If you want for about $1500 you can turn the z into a 50 h.p. Monster. I only say this because the drz has a ton of aftermarket parts. you can get anything for it. Lots of people do long highway rides on them, geared right itll pull 100 mph and cruise at 85 all day/night.
Before you start moding any bike do research and ask questions in the bike specific threads, learn from others mistakes.
whatever you choose, have fun and might I add take a msf dirt bike course.