The KDX has a reputation for reliability. I've got one in the garage because I much prefer to be riding than fixing things or waiting on parts. I can honestly say buying it was one of the best decisions I've ever made.
The only real fault that I can think of is that mine will leak gas from the carb overflow if it is leaned over more than normal on the kickstand (on uneven ground, for instance), which seems to be a somewhat common complaint. It might just need a new float valve needle, but screwing around with carbs isn't my favorite way to spend an afternoon. I just always turn the fuel tap off when I stop. Maybe I'll dig into it one of these days.
I swapped the piston out for a forged one shortly after I got mine, but I'm not convinced that it was necessary. The old one looked to be in good shape, and it was the original piston according to the previous owner. The procedure is fairly straightforward, although it took me most of a Saturday, as I recall.
Throw some wraparound handguards on it if it doesn't already have them, and carry an extra spark plug with you.