I'd also recommend a used 250 non-race bike, but make sure it's not super tall at the seat, or running a close-ratio transmission.
The KLX250S has adjustable suspension, 6spd, and a simple carb. Beef up the subframe if you want to haul cargo on it. It's not quite as tall or heavy as the WR250R. It also tends to be less expensive. Beef up the power with aftermarket parts as your abilities grow. A 351 big-bore and a pumper carb with exhaust can give it some grunt.
Simpler bikes to consider would be the XT225/250, KL250 Super Sherpa, KLR250, and CRF230L.
The DR-Z has a close-ratio 5spd and weighs a bit more. The WRR is tall, expensive, and heavy for a 250. The CRF250L is heavy for a 250, and may not be available in large numbers used just yet. At a strong 33" inseam, you might have the leg to hold/lift them all without issue though.