An old, air-cooled dualsport would be my recommendation. You're big enough to handle something like a DR650SE, XL600, or KLR650 if you're starting out in the dirt, and they don't really have enough power, or shocking power delivery, in stock form to be crazy on pavement. The power delivery of a 2-stroke 250 dirtbike, coming into the powerband, can easily put you on your butt...or worse. Even a 250 dualsport can put you into a tree at high speeds, but the delivery is usually a bit more manageable than with a plated 200cc+ enduro racebike. Take a BRC to learn basics, then practice the drills and ride some trails...until you can control the bike without having to think about it while you negotiate the crazy drivers in traffic.