What would be some of the criteria you should employ? Well, it seems to me that they should (i) either be piston fillers that draw ink from an inkwell, or fountain pens that allow you to use a converter. The first are few and far between nowadays, the second are plentiful. Almost every fountain pen that uses a cartridge can also use a converter. And (iii) it should be a fountain pen that allows you to change the nib or nib section easily and without any fuss, so that you are not stuck with the nib you with which you bought the pen.
On that last item, some very good suggestions are made. Worth checking out. Especially if you are new to the world of fountain pens.