I guess I should take my cue and jump in here.
I have a few of the ArtPen, but none of the "lettering" variety. I believe it's a round nib that comes in a variety of widths. It should be similar to the Speedball "B" series pens. They could be ovals like the Speedball "D" series pens; I just don't know since I don't have any and have never looked at them closely enough.
The "Drawing" ArtPen and the "Calligraphy" ArtPen are basically the same pen, but the "Drawing" nibs are standards (and come in EF, F, M, and I presume B but I've never seen one) and the "Calligraphy" nibs are either stubs or italics. I'm not sure which they are. The EF absolutely puts down a finer line than the 1.1. I think the M puts down a finer line than the 0.9! Indeed, the M puts down a line about the same weight as my Rapidograph #2, which is supposedly a 0.60mm.
The ArtPen is a fairly inexpensive pen. I'd definitely get a couple of those before having a Pelikan modified. If nothing else, you'll have a better idea of what your needs are after using the Rotrings.