Talking as a ex-motocross racer I suggest nothing over 250cc Four Stroke. They are the WRX of the MX scene. A 450cc will pull your arms off and be very hard to control, thus you will not have as much fun.
The 250cc four stroke will allow you to enjoy the ride with more power than you will ever need. Trust me I used to race a YZ450F, great bike but a real animal of a power band.
My pick of bike would be a Yamaha YZ250F 03-04 as I have owned 2. Also look at the Kawasaki KX250F 04-05 had one of these too. All these bike should be around the 6K mark.
Buy private and save over $1500 from what the bike shops charge. Check the Quokka.
