It sounds like the only forces you have to take care of it's the portal itself (if you're aiming for a portal design, that is). Check the DIY-router forum and its subforum or the Photo gallery to see different ideas how people have made their own designs.
If I were you, I would go for two "raised" rails to keep moving things away from children fingers etc in the table level, as they seem to find all the places were they shouldn't put their hands... Steppers or servos doesn't really matter I think, you have no cutting forces. Also, I would drive the gantry with a belt on both sides having an axle in the rear end across the table running the two belts. I'm an EMC-fan and EMC is a great controlling software. If you're not afraid of Linux and some manual tweaking it's awsome.
Something like this is what I have in mind:
Sounds like a fun project! Hope you'll get it up and running.