You mention that your giftee will be signing many documents. If signature is the main use, you might include a couple of different nibs with the pen. There are various widths and even calligraphy nibs. You can create your own special signature by changing to a different nib and practicing a little. I liked using a fountain pen because it sort of "forced" me to write more slowly which gave me time to think about what I was writing. I enjoy writing personal notes and people seemed to respond positively, knowing that I had taken time to write a note which was obviously personal. Of course, there is a time and place for the speed and simplicity of an email, but I think there is also a time and place for a handwritten greeting.
My favorite pen is a Parker Duofold International in Pearl and Black. They have been out of production for several years, but there are plenty on ebay. You didn't mention price point. The Duofold must be pretty collectable because they have doubled in price since I bought mine, but you don't have to spend big bucks. Levenger makes several attractive and functional pens.