The KDX reccomendation holds for a novice rider. If you decide that off-road isn't for you, you will be able to sell it and get every penny back. If you want to move up to something else, you can sell it, get every penny back and get something else. In terms of just having a good time, the KDX is hard to beat too. You can ride the pee out of it, and probably not scare yourself. Suspension is a bit rudimentary, but workable. Once I beefed up my fork springs and did a little revalving, my KDX stopped trying to smack me in the back of the head on downhills. The short wheelbase can make them a bit of a handful on steep climbs too.
Barnyard's reccomendation is good. If you think you are going to be riding/racing off-road for the forseeable future, step up to something that is going to work better for for you over the long haul.
2013 KTM 350EXC-F, 2005 BMW R200GS, 1984 Kawasaki GPz 750