I would splurge on the Herpstat. It will be the BEST investment you ever make for your snake. You only need 1 UTH and the small should be fine. You just need it to cover 1/4-1/3 of the tank floor to create the best gradient.
I think the 10 gal is fine until he gets bigger, but definitely consider a T8 or something very similar to that (I think Vision makes some similar cages)
I'm not a fan of the repti-carpet, but some people swear by it. For bedding, most people use Aspen. I prefer cypress, coco fiber, eco earth, or a blend of the three. This will hold humidity and help you maintain your humidity levels especially when your snake is in shed. You only need 1/2 in-1 in of bedding or else your UTH is useless (because it won't penetrate the bedding).
The probe on the thermostat will be placed directly on the UTH between the UTH and the glass. To measure temps, you will need an infrared temp gun or two DIGITAL thermometers with probes. Walmart sells some that are about $12 and work well.
As for ambient temperature, it all depends on how cold you keep your house. You may need a CHE in the winter if you're like me and don't like to run the heat. However, in the summer, my ambient temp is 80* ish so a CHE isn't necessary. As long as the ambient temp is over 75*, you shouldn't need an additional heat source.