| T hermostat will only measure where it is. If it is in the tank, and not the click-clacksthere could be quite a temp difference. If the radient heat is going directly into the click-clack then it will be like a car in the sun 40 - 50 C while the enclosure might be at 30. Remember that you can get it all set up prior to getting the snakes, and test out the temps in and around the Click-clacks.I would consider spending a week with empty click clacks and plot temp max min in each. Read the breed specific details on optimum temps and required ranges. There should be stuff already on the lists (do a search).
Go get a dual thermometer unit (australian geographic/ Jaycar/ Dicksmith/etc) this will record max and min. Should be able to get one of these fairly cheaply.
most of the hardware you get eill last a fair time. Depending where you are and whether your place is A/C then Cooling might be more important than heating (FN QLD Vs Melbourne). A heat mat is inexpensive, and even if you don't use it in the short term, you will eventually.  | 
