in my experience it's all the same really. even intuos 1 should still be fine. i used to use a series 2 until last week when it broke. as long as you are comfortable with the pen shape and have no need for the tablet-buttons or the context menu stuff (all disabled on my series 5 here anyway), you can go with whatever model…
yes, you can use the same nibs across all of the intuos pens. as for the usb port - careful with that, it's in plain sight on the 5 medium where the arms tend to move around a lot and the connection feels a little wobbly on both of mine out of the box. i don't understand that choice of placement at all when they had it out…