I need feedback from those of you who have variable power supplies.
What voltage makes a difference from "standard". My power supply is at 20V

Buying one that ranges 15-24V will be ok you think? Will the wild cars be tame at 15V or should the range start lower you think? The obvious is the more choices the better obviously but I have a good deal for this one, its powerful and affordable.

Will I be needing lower than 15V you think?

15 to 24 is ok unless you want to run G Jets...
Those run at 12 volt...

It really depends on the size and layout of your track, the type of cars you are running, and the controllers you are using. You can add variability to some of these parameters to tune the racing experience to your liking.

For voltage variability, ideally, I prefer to have the ability to tune it down to 10V on the low end and up to 20V on the high end. Drag racers like to go to 24V on the high end. Any variability in this area will be helpful.

For controllers, the traditional resistor based controllers (e.g. Parma) offer the least amount of variability. They tend to work well for one or maybe two types of cars and not so well for others. Their behavior is closely dependent on the current response of the armature. Cars that draw a lot of current tend to have a dead band on the bottom end of the throttle and cars won't move until the throttle is mashed. This would include cars with heavy traction magnets and low resistance armature windings. Cars that draw little current (like AW TJets) tend to be too twitchy and fly off the line with little throttle. Picking a single resistor value is always a tradeoff if you run more than one type of car. Putting an additional inline potentiometer, aka "choke," on these controllers can help tame the overly twitchy types of cars, while adding variability with additional series and parallel variable resistors can help get rid of the dead area on the higher current cars. (Siberia Racing describes some ways to do these types of custom mods to a resistor based controller.)

Electronic controllers, both diode based controllers like the Professor Motor and Omni, and transistor based ones like Difalco allow the voltage output from the controller to vary independent of the current draw. This makes them usable over a much wider range of car types. Adding variable chokes, brakes, and the ability to adjust the input to output response curve, aka sensitivity, makes them even more flexible and allows you to tune the controller to get the desired response from the car.

The combination of a variable, adjustable controller and variable power supply will go a long way towards getting control over your racing experience.

Designing a track with large free flowing curves and long straightaways will help too, if that's what you want.