www.bikepoint.com.au
have you riden a 450cc or 250cc two stroke motorcross bike? Do you think you really need one? they are pretty zippy...2 strokes are better value, cheaper and just as quick.
for 6K look at any 450cc MXer, probably round 00-02 models...
edit, are you after a quad or a MX bike? All above relates to MX bikes..