How old are you? How much dirt experience do you have? Do you have to ride far to venture off road? Do you want to tour over nite or just do day rides? Are you going to far flung places or just down the road a spell? Do you want to ride fire roads or venture farther off road into single track? Whats the most you think you want to spend?
Cant really answer your question properly until I have some sense off what you mean by dual sport, which the following questions help narrow down.
I have a smaller 300cc dual sport, a vintage 750cc scrambler, and a larger 1190 Adventure bike, and Ive ridden several in between bikes, across multiple types of trips, from 30 mile single track, to 300 mile trails to 1800 mile adventures, all in a wide variety of roads.
Heres what I can safely say without answers to the above: you can get by on a WR250, youll enjoy a DRZ400 (unless you have a short inseam or a wide butt), youll LOVE a KTM 690R, you probably dont need anything bigger than 800cc unless you only have one bike, and...sorry if this last part is controversial...dont waste your time with a KLR, which does nothing particularly well but is better than nothing at all, and which Id only ever use if one were given to me and I had few other options.
But adventure and sport is where you find it so whatever you end up with, just get on it and get out riding. and if you plan on venturing to more than just a few local dirt roads, take a class if you dont have previous and recent dirt experience. Youll save a lost of time and frustration and shorten the learning curve.