You would have to contact Reptile basics, as I have done all my own wiring, as far as getting the flex Watt pre-wired.
Yes 400 Watts or 500 Watts will be more than enough to handle the approximately 100 Watts. Ideally if you use the proportional T-stat the only time it will see full load is during start up or after a power outage.
The Herpstat ND adds other features other than the night drop feature which some people find useful. It adds a high and Low alarm that can be set in 1/10ths of a degree from your set point. This means if you set the High alarm to 2 degrees and the low alarm to 4 degrees and set the actual temp on the T-stat to 90 degrees if the temp ever exceeds 92 degrees the herpstat will beep out an audible alarm and if the temp drops to 86 degrees the same will hold true. Issues have come up in the past where the temp probe was knocked off the heating element and caused the T-stat to turn full on. With the low alarm set above room temperature you actually would have a chance of discovering this shortly after it happened instead of after it cooked your snake. A nice option, but most of us keep an eye on the probe when we slide tubs in and out anyway which means we would see it. Then again if you are in a little hurry you might miss it!
If you are tight on budget though my opinion would be the basic herpstat is better than anything you could find for less than its cost. Don't need to be spending money you don't have if you can avoid it, while still keeping a good level of safety for your snakes.