The main trouble with using fish tanks for reptiles is the obvious lack of ventilation, if you have to use the fish tank, definately styrofoam would be best for heat insulation, and make sure the finished tank is in a place where there is a bit of a breeze, so some air can get into the bottom of the tank. If you were going to make the sides and back with timber panels, maybe think about making a bigger enclosure with mellomine or ply with vents, then cut the glass out of the fish tank (separate it into its 5 panels) and use the front and back glass panels as sliding front doors? You can get the glass track from bunnies. Just make the woodwork to suit the size of the glass?
Where mellomine is better than mdf, is even tho they both use nasty adhesives, mellomines plastic laminate outer panels keep the nasties inside, where mdf (especially if it gets wet) tends to continuously shed dust and therefore adhesive particles.....
all just my own opinion