"Do you totally build the cage, put the tracks in, and then put the glass in? or is it in another order?"
No, that's basically how you do it. You might look at the FineGTPs.com site to get a better idea.
"Also, the cage unit i am buiding is going to be on casters, yet it will have insulation. Should i cut holes in the insulation for the casters? or just attach them to the insulation?"
You'll need to cut holes in the insulation big enough so that the casters have room to swivel around.
"the insulation will be polystyrene, what rating should i use. (my room temp goes from 78 summer high to 63 winter low)."
I would question whether or not you really need insulation on the bottom of the cage. My best guess is that it won't do much unless you're using a UTH under a floor cutout or a thin floor made from plastic or something else. In that case a piece of the foil-covered styrene insulation over that area might be a good idea.
But better yet, put the styrene insulation *inside* of the cage and cover it with vinyl flooring or something else. I did that with my bearded dragon cage, but more to raise the floor rather than to insulate it. This may not work as well when UTH's or heat tape are used.
But all these these are mute points as I don't know the following.
1) What species is this cage for?
2) How large is the cage?
3) What is the cage constructed of?
4) What is the type, placement, and amount of ventilation in the cage?
5) What heat sources will you be using for the cage?
Regardless, the styrene insulation will primarily function to block radiant heat. This is why the foil covered type is best. Insulating reptile cages is a bit different from insulating a house where lots of convective heat sources are used.
In this regard the R value or thickness of the styrene won't have as significant of an effect.