For something like this I would use my AXYZ CNC Router. MDF is messy to cut and you'll want to ensure that you have good dust collection. Most CNC routers would cut the material, but some may need to make several passes to avoid slowing the bit down too much and burning it up if the spindle isn't strong enough. As far as hold down when cutting large letters, I simply screw the sheet to the MDF router table and the letters are heavy enough that they should not move.