Does it come complete with the double sided ink pad and ink for $4.94?

The ink is self contained inside the body of the "self inking" stamp's body. It passes the lettering of the stamp by the ink on the way tot the paper when you press down. What you see in the picture is what you get, and it should stay inked for a while. No extra pads or ink came with mine.

How many characters can a line fits? I need this for return address and I need 28 characters (already including spaces) for my first line ?

On the one I ordered the longest line was 19 characters, including spaces. The font was tiny, the font was about half the size of a normal printed return label.

What are my font choices?

Can you specify a font AND SIZE? I need just one line at font no larger than 1/8"

What size is the mechanism (not the imprint size)?

When set up with the imprint mechanism down, it is 2 1/4 inches wide, 3 inches tall, and 1 1/8 inches deep. When pressed to imprint the height is 2 1/4 inches.