Rebekah took her WR250R to baja with us a few years ago before getting her FE350 and Eric has done a number of trips on his KLX250R with a big bore kit. Both seem to do good, but cant say how they would do we someone of my weight. Havent ridden either.
My XR's motor/exhaust is stock other than jetting. The suspension has a bit of work, plus the requisite armor, bark busters, big tank (5.8 gallon), pegs, fat bars, race light etc. It weighs a real 294 lbs with two gallons of gas, so its not exactly a lightweight but not crazy. If you are really curious about them, you are welcome to come up north and ride it.
After riding the XR for a bit, it would only take a 5 minute spin on the DRZ onverthe pavement to see what I am talking about. The DRZ is just so much smoother. It can be made to perform much like the XR offroad, just a carry a bit more weight. But if its an adventure "light" bike... well you are going to be adding luggages and spares anyways, so geeking about weight is really kinda a mute point. If you are really looking light weight dirtability, get a plated dirt bike. But a plated dirtbike wont make a good light adventure bike.
Important note about the DRZ400S. It got the fully adjustable DRZ400E's suspension in 2002 I believe. You want/need this. Google to confirm year the change over occurred.
I've also owned and spent considerable time on a XR650L and DR650. The XR650L will chug along at 65-70 on the highway with a bit less fuss than either of the 400's. Its pretty decent offroad with some tweaks if the rider has some experience and can deal with its height and 360lb wet weight when kitted (mine). The DR650's motor is a real gem. Much smoother than the XR's with the same ability carry a moderate highway speed. It also runs cooler that the XR650L. The achilles of the XR650L and its older design is heat... it breaks down the oil quicker, and they tend to burn a bit more than the DR650s if run all afternoon on the highway. Its what often leads to the premature engine failure of them. For the average dualsporter, non issue. As an adventure light bike, it something keep in mind if you were doing cross country/state work. Non issue if aware and keep on top of it or even add an oil cooler.
The real gem would be the DR650's motor in the XR650L's chassis. I swapped back and forth between the two that aquired and kitted out both, then spent 6weeks riding to/from and in baja from the city giving me the ability to make a pretty fair evaluation of them. :teeth
I would never consider such a trip with that much road work on the XR400R, but might on the DRZ400S.
Money no object, probably the best bike for what you are trying to do is the KTM690 or Husky 701. But you probably already know that.
Or get a damn dirtbike and a KLR for your adventures.... :laughing