You should get a thermostat no matter what you use... rack or cage. You can get by with a rheostat (dimmer), but a thermostat is much better.

Helix and Herpstat make the best thermostats for reptiles. They usually have one probe (Herpstat makes some units with multiple probes), but all that is needed is one probe/thermostat per rack. You can place the probe in a number of places, but people usually adhere them directly to the heat source.

You pretty much covered all that is needed for the rack, although I would add a digital thermometer ($13 Accurite from Walmart). Although you snakes don't require light, a clear tub such as the Sterilite 1756 would be better as you can monitor what is going on in the tub without pulling it out.