With your street riding experience and size, I think any of the 450cc MX bikes would be a thrill for you, and you could probably get a 2007 or 2008 in your price range. The power on these bikes is more than you need, but you've probably learned throttle restraint from you street bikes.
The Honda's seem to be the most popular 450s at the track, and they are fine on the trail, too. The modern 250 two strokes are lighter and still plenty powerful, and should be less expensive. For a trail bike, the KTM's are highly regarded, and they also make a 300.
The 250 four strokes are a blast to ride, and worth considering. But if you are used to big power, might as well go the 450, IMO.