I have heard bad reviews on the exo terra incubator.
Yes, the fish tank tube heater is the one I was referring to. Get a good one, as stated above the cheap ones can fail. I however do not agree that all of them are unreliable. There are quality ones out there that I have trusted to keep reef tanks warm that had more money in fish and corals than any amount of eggs I have incubated.
As far as temp setting, that you will have to play with. Try setting it at the temp you want as your ambient. Let it equalize the check ambient. Raise or lower from there. The nice part about using water and a water heater is that it helps with humidity and having a large body of water as your heat source will keep temps very consistent.
Vermiculite can be purchased at most stores that have a garden center.
Like I stated above, this is the most cost effective way that I have found to build a reliable incubator. If this is potentially a one time thing for you, it is a great way to get started. If you are planning on building a business breeding high end snakes on a large scale, this is not for you. If you want something a little nicer and have more money to spend, the wine coolers make awesome incubators. Just make sure to get good heat panels and a good thermostat. There are also a lot of you tube videos showing how to convert them.