My current setup is a ZooMed UTH. The one rated for 20 gallon enclosures. As long as you have some sort of regulation, then the "who" of the UTH is less important than the "size" of the UTH. Look for something that will fit 1/4-1/3 the bottom of your enclosure. That seems to be a good size coverage to give a nice temperature gradient between the warm and cooler ends.
As for t-stats, probably the most common suggestion will be to grab a HydroFarm from amazon. They're probably the best price to performance. There's more expensive ones on the market, with features like proportional temp regulation, night drop settings, and even some even more expensive thermostats with multiple probes to monitor more than one enclosure/heat source.
Finally, as far as thermometers go, the only real *must* bit of advice is make sure it's digital with a probe. Analog thermometers and the sticky strip thermometers are both notorious for being incredibly inaccurate. ZooMed, Zilla, and a billion other companies all make $5-$10 digital thermometers with probes. If you wanted to spend a smidge more, Accurite makes a ~15 dollar indoor/outdoor with humidity model that is also really popular.
When it comes time to set it all up, you will want your thermostat's temperature probe to be in the enclosure and directly above the heat source. That means on the glass/pvc/wood, UNDER the substrate. Having another thermometer's probe in the same spot to assure accuracy is always a plus. No matter what you use for bedding, you snake can and will find it's way down to the actual floor of the enclosure. You don't want to risk your snake burning it's belly or causing any biological irregularities from exposure to temps too high.