The DR-Z will give you the most bang for the buck of those bikes listed in stock form. Aftermarket support is huge. And while it's far from a racebike, you stand the best chance of compromising with it, versus the others on your list, should you choose to compete on it.
And no, I don't own one.
'07 DL650, '06 KLX351, '97 DR350, '83 XR350R (vintage GP racer), '74 Bultaco Alpina (trials)