List of things to buy or make
I would hold off on buying the heatlamp until you've tried the heating pad out by itself. If you're using rubbermaid, and since it is summertime, that may be suffecient all by itself...depending on the size of the cage and the ambient temps of the room you keep it in.
As for the substrate...newspaper works just fine, or papertowels. There are plenty of others that give a "prettier" look, but if you're going for simple and inexpensive, then paper is the best.
Be sure and decide what you want to use for a waterbowl. It should be large enough for the snake to get completely into for a soak if it wants to, and heavy enough that it won't tip over when the snake crawls over the lip.