The XR400R would be the "easiet" to learn on, and still PLENTY powerful. The hardest to start. The heaviest to drop. More of a trail bike.
The 05' YZ 250 is a good price.
And really all of the major brand bikes are fine. It's more personal preference than anything. The biggest thing is make sure the bike is clean. If it looks really filthy, its probably really neglected. Dont be afraid to pick up a compression guage and request a compression check on the top-end if its a 2-T. Also, dont forget to look at the front forks. If they are oily, the fork seals are leaking. This sucks. Look really good where the bike was sitting for any stains (Gas, Antifreeze, Oil), make sure the engine is cold to the touch when you first look at it. Be sure that it starts easily ( A two-stroke that is jetted right should take no more than 3 or 4 good kicks. Mine always go on the 1st or 2nd) Once it starts, let it warm up to full operating temperate. Make sure that both the front and rear brakes DO work. Take it out for a spin and shift it up from 1st to 5th (Or 6th if it has it) gear several times to ensure that it shifts smooth. Once you know that it is fully up to temperature, lean forward a bit, get it in 3rd or 4th gear, and run it wide open throttle to redline to make sure the engine runs strong/no odd noises. Now bring it back to an idle. Make sure it idles and doesn't want to stall. Its no fun learning on a bike that you have to give throttle to keep running ...
Check to be sure there is still adjustment left on the chain and clutch. Check that the tires have good knobs left on them and its not all rounded off and ****. And pay attention to ANY weird noises or vibrations. Check the fluid level(If a site glass is present, most 4 strokes it is ) for the oil, etc.
I've found that the cleaner a bike is, the better maintained it is. The guys that are anal about their bikes looking good, and usually the ones who are anal about them running well. They are the ones that throw money at every thing when it goes. And this is the type of anal retentive person you want to buy off of.
Be sure to find out how fresh the top-end is, any reciepts they might have for any work that was done. And find out, FOR SURE, what ratio they mix their oil/fuel at, which fuel they run it on, and what brand oil they have been using.
Good luck man.