I currently own a 2003 KX/KDX hybrid, but also own a 2004 KX 250, 2005 KX 125, and a 2007 KX 250. Yes, I am in Dirt Bike Buyers Anonymous.
I think most of the latter years changes were made in the 250. The 125 didn't change much, and the later year bikes (after the stopped importing them to the US) would have been just repeats as well. Anything from 2003 and up should work fine for what you want to do. I would focus more on getting a quality bike for a good price.
There is an excellent KDX only site called
kdxrider.net that has a whole section on hybrid builds. Basically you will need to trim your swingarm a bit, fashion some new motor and head stay mounts, fix your airbox with a plate (to use a KDX boot), and modify your exhaust. The KDX motor is a lot shorter than a KX, and comes up short of the front motor mounts. It sounds like a bit of work, but it really isn't that much fabrication. What you will end up with is a perfect trail riding/enduro machine. I absolutely love mine, it's a blast to ride and has torque for days. Have fun and post lots of pics.