ackies dont require any UV light at all, but you do need to dust they're crickets with a quality supplement. i use miner-all. for ackies a 3ft. x 1.5-2ft. cage is fine, just make sure they have about a foot of dirt to burrow in once they're full grown. the general rule of thumb for monitors is the enclosure should be twice the length of the animal and the width should be as long as the animal.
i personally would go with an ackie. they are a great, fun to watch, active little monitor and i dont think a cham would be near as much fun, but more of a display animal.
EDIT: also, if you plan on using a aquarium with a screen lid you'll need to cover most of the top to keep heat and humidity from escaping.