I would recommend heat mats if you can't afford radiant heat panels. You have to completely seal them from the inhabited space. Glass, tile, vinyl, they all work fairly well. You could also use back heat which involves running a big piece of flexwatt down the back of the cage. This requires a thermostat.

Accurite weather stations are a good option for measuring temps. A temp gun would also be nice.

I would not recommend using only a dimmer. A thermostat really is a must.

I am not sure how you are constructing this, but I wouldn't use screen at all if you can avoid it. You could use some small vents or drill some 1/8"-1/4" holes on the sides.

Since most snakes are nocturnal you don't really need lights. If you want some viewing lighting you could use cold cathodes or led lights.

Good luck!