The top 2 bikes for a Baja prerun/expedition bike would be an XR 650R (owns baja) or a KTM 625 SXC (light version of the rally bike that owns Dakar).
Out of the bikes you listed you want a XR 650L, or the DR650 I would say, probably with a shock/fork upgrade.
My simple breakdown....
XR 650L, rugged old school air cooled DS bike, I'd ride it anywhere. Could use a fork/shock upgrade though.
XR650R lighter, faster, water cooled, better suspension, but has no e-start, and has a weaker aluminum subframe, not meant for luggage and will break offroad if loaded. Just needs a bigger tank for running Baja.
The DR 400 is fine, until you need to run a highway at over 60mph. It's also not really lighter than the 650. Sips gas compared to a 650, a 5gal tank will go a lot farther.
KLR will do it, but it's heavier and will break things, mostly the cheap bolts. Upgrade most of the load bearing bolts to good grade and you'll be ok. Also needs a fork/shock upgrade.
DR 650 is a good bike, I'd ride it anywhere, needs a fork/shock upgrade though.
650 Dakar, too heavy, expensive and you'll tear it up prerunning baja. No way I'd want to even try that bike. Might struggle through it, but the bottom of this list.
The KTM LC4 enduros (NOT the adventure, look at the SXC) are lighter, built stronger and are faster than any of these bikes but maybe the XR 650R(about the same as the DR 400), have e-start, moly steel frame/sub frame to pack a load, good WP suspension and would do about 200 miles with a 5 gallon tank that you can get from the factory. But they don't sell them new in the U.S.A since 2004 and they aren't cheap.
The smaller KTM RFS based bikes are a little over 50 lbs lighter, but I'm not sure of carrying luggage on them, 5 gal tanks are available though.
If ya like, I just rode about 700 miles 90% offroad on my KTM SXC along with two friends, one on a DR650, the other KLR 650 you can read about. Both of em over packed IMO though.
http://www.twtex.com/forums/showthread.php?t=18205
