My wife had a 2008 135i and we went to buy an F30 to replace it. It didn't work out, she felt like it was too big and too soft, so we started out looking at the X1 and X3, but of course she also felt the X3 was too big. To be honest, from my perspective, the X1 feels like the 135i in nearly every driving manner, but with the more neutral handling of the E90. No surprise there.
I worked in Frankfurt for too long to think of Mercedes as anything but taxis, even though my dad's driven different ones for the last few decades. We considered "the hat" as my wife refers to the GLK, but it felt even older than the X1 and just didn't feel special. The ML is certainly nice, but the new one is so much better done than the used one you're considering, that would be a hard pill to swallow.
Of course from reading your post, it sounds like you're the opposite of us, looking for something bigger, rather than smaller, and perhaps more luxurious rather than relatively spartan. The X1 fit our needs perfectly, but I think it might not be the same fit for you. Be sure to test drive both the 28i and 35i, we were convinced that the 28i would be better because of the newer transmission and great write ups, but we ended up prefering the 35i for the sound, larger brakes and small price differential.