I agree with Deborah 100%. Get a good thermostat. With a rack like this one, you could actually get by with a Herpstat 1. Research and talk to people with snake racks about how they have their heat tape and probe's set up. With one snake it's very simple. With more than one, it's still fairly simple, but you will want some more thermometers and a good heat gun is a good investment. This will likely be your single most important and most valuable purchase, so considering investing some time AND money into it. It will pay for itself 100x over in the long run.

If you bought an economy style rack like the one above, I would consider dark paper/cardboard/insulation board/etc... or something to cover the sides and back. This will serve to contain warmth, and offer your snake some more security. If you do this, one hide per tub is plenty. You can get nice hides and crock-style water bowls from Reptile basics, or you can make your own hides and get some crock-style water bowls at the dollar store or Walmart in the pet sections. Substrate, I'd use paper towels for the 6 and 15qt size tubs, and look at kraft paper for the larger tubs. You can buy it precut or by the roll. These are both cheap, easy, and let you easily see what's going on with your snake (pee, poop, urates, mites, etc...).

The last thing you MIGHT need is a space heater for the room where the rack is going. When I built my first rack, I didn't think about ambient temperatures. Even with heat tape, your rack will stay at your ambient room temps. maybe a degree higher. If that isn't warm enough, something like an oil filled radiator style heater is great.