The thermostat probe goes directly on your UTH outside the enclosure.

A temp gun is great for people who have rack setups, but if you are going for a single setup I wouldn't bother with a temp gun. Head to the home depot, lowes, or walmart and pick up the accurite weather station indoor outdoor thermometer. It costs about $12 and is the only thermometer/hygrometer you will need in a single setup.

You don't need to really "invest" in a paper background. I like using black paper or black foam core. This doesn't need to be expensive. You could even print your own background off the computer.

How big is this enclosure? If you plan on getting a baby snake then you will want to go on the small side.

A wet towel on the screen is not a good way to keep humidity up. Actually, it is a pain in the butt. You need to replace the towel daily to prevent mold growth and its just not really that fun to wash a towel every day just for a snake enclosure. Covering 3/4ths of the screen top with aluminum tape is a much better, more permanent solution.