If I where to buy a mostly stock bike to learn on and putt around the trails with I'd look at:
Honda XR250 or XR400. They're heavy and slow compared to a modern 450 motocross 4-stroke, but don't require near as much maintainence, and they make tractor like power down low which is nice for cruising around the trails.
Kawasaki KDX200 or KDX220. Pretty popular 2-stroke trail bike that shouldn't be too much of a handful for a beginner.
And of course there are tons of less-than-fullsize bikes out there that are pretty nice for learning on, but once you figure everything out you'll probably want something a bit better anyway.