Maria, somewhere on here I've posted a review of the Derbi, in fact I've raved about it across numerous posts, across numerous forums now I come to think about it, so you can guess that I'd say buy one.
For preference I'd recommend the Adventure, you get the 21" front wheel, better suspension, a bigger front disc and the tall front screen and personally I think it looks better.
You mentioned the Mongol Rally, oddly enough a few years back someone did it on one, there's also a guy called Fernando Retor who has travelled across Europe several times as well as venturing into the Sahara, and a bunch of Singaporean owners who regularly venture into Malaysia and beyond. It's only the Anglo-Saxons who seem to reckon a 125cc is far too small.
There's a small bunch of owners who frequent the Adventure Bike Rider forum, and a couple of Facebook groups, and a Spanish-speaking owners forum. And if you're in or near London feel free to get in contract if you'd like a look over mine.
Happiness has 125 cc