For strict dirt riding, a teeny bike like the yz125 is ideal--do a few weekends at Hollister on it and then try something a little bigger. This one is ideal if you want to move from, say, street riding to doing weekend bike camping trips or fire/gravel road riding, with some trails and mellow singletrack mixed in. The DR is geared low, with a 6-speed gearbox, and has 11.4" of clearance, so plenty dirt capable, but it is 310 lbs.
Can't comment on the sumo trackday, but the front is a 21" and the rear wheel is an 18-incher.