If you are just starting out, are not physically huge, and plan to do more difficult trails and such, I might even suggest a 125 or 150 (Yamaha has a nice TTR125 set). The ability to always get a foot down is very helpful.
Personally I run a Honda XR250r on our trips because it is large enough to go on the road if necessary (plated, will do about 65 to 70), its dead simple (air-cooled 4-stroke single), and fairly light so I can get it up and down from the rack on the back of the van without too much pain. Also parts are plentiful and cheap.
I do miss having a magic button, especially if I have flipped the bike upside down and have to kick it for 15 mintues to get it firing again, and some of the newer design water-cooled bikes are lighter, but they introduce complexity and tighter maintenance schedules too.
-Erik