I will agee with the other posts a KDX will make a fine bike for you. My Father and Uncle both ride KDX's and they both weigh over 200 pounds. We have put heavier springs front and back on both bikes, also I installed Race Tech valves in both forks. The valves and the suspension worked better on the lighter sprung bike.
The problem as I see neither of the bikes have as good of suspension, or motor as my RMX even though it is a 95. (which I would be glad to sell you)
All I'm saying is that I would open my search a little. A KDX would be a good bike, a used RMX would be a good bike, you might even find a XR 400 to suit your needs.
If I was buying a used bike I would go more by how good of condition the bike was in and how good of a deal I was getting, then just one particular model.
Be careful new plastic dosen't mean the bike is in good shape! Check the compression, how well it shifts, does the clutch pull easy and let out smoothly. Put the bike on a stand how tight are the wheels, wheel bearing, also is the rear suspension tight? grab the swing with the bike on a stand and see if it moves side to side, or up and down. It should go up and down of course but it shouldn't clunk or click.
With the bike on the ground grab the front break and roll the bike forward and backward and see if there is any play in the steering stem bearings.
Don't get me wrong almost any of these problems can be fixed but you won't know how bad the problem is until you take it a part. For instance you might think it just needs some new rear wheel bearings, but then you take it apart and find out the hub is shot, now the price of the repair has just went 10 fold.
Also look for rounded off nuts and bolts, this is a sign of a bike being wrenched on a lot as well as the quality of the mechanic/owner working on the bike.
Another tale tell sign is look at the pipe mounts, a lot of hard riders crack or break the chamber mounts.
Even though I ride and own a KTM, I would pick a good used KDX, RMX, or XR over a clapped out KTM or any other brand/model.
Happy Hunting,
DT.