Yes, going with one of those TTR/KLX/DRZ 125/XR100 deals might seem like a good idea at the time, but you will outgrow them rather quickly.
My advice: Try out a 2-stroke MX bike. You may say that you don't want one now, but 95% of my friends and everyone that lives here (we ride pretty wide trails, opened farms, dirt roads, construciton sites, and occasioanl tracks) all said that they didn't want 2-strokes at first until the first time they rode someone elses and experienced them for themselves. Within a matter of 6 months, nearly everyone down here has a 2-stroke MX bike now from 80s, 85s, 100s, to 125s.
I would look into getting something like an 80cc to 100cc 2-stroke. They are a lot of fun, and dispite what some people say, you really don't have to ride them specificly down at the tracks. If you get a Suzuki RM or Kawasaki KX, they have more low end power than the Yamaha YZ and Honda CRs, so they will be more suitible when it comes to trail riding. I have ridden both CR 85s, and KX 85s around trails and farms here and I can say that the KX would be my choice if I was getting one. So if you ever decided on a 2-stroke MX'er, I think you would do fine. I ride a KDX 200, which is also something I would DEFFINETLY look into if you think its something you would want. But the MSRP is 4,000$ - Around 1200$ more than the 85's. Its a little bigger than the others, but its not meant for jumping and lacks the suspension of the MX bikes. (however, the bike is very mod friendly for if you ever wanted to mod it into an mxer)