Picking an amplifier is mildly tricky because there are so many variables to consider. The first issue we encounter when outfitting a Dodge Challenger with an aftermarket amp is compatibility with the stock head unit. The Challengers use a CANBUS digital audio signal which is a totally different interface than a traditional RCA line level input/output. Thus, an adaptor will need to be wired in (not that this is complicated, it is just something that needs to be noted). The remaining variables to consider are channels, impedance, and of course, power output.
Amplifiers come rated with a certain amount of channels, essentially signifying how many speakers it is able to run. For every speaker you want to run off the amp, you will need a separate channel. For example, if you want to only run the door speakers off an amp, then a 2-channel stereo amp would suffice. A 4-channel amp is the most common, of which is capable of driving 4 coaxial speakers. What if you want to power 4 speakers and a sub? Well, in this case you could buy another amplifier specifically for the sub. Or, there are some voodoo tricks you can use with one amplifier in order to increase its total connectivity.
It is possible to bridge the channels of an amp together to power something else. For example, you could have channels 1 through 4 each powering a set of speakers, but then have channel 3 and 4 bridged together and also powering a sub. Cost per watt in this case would be the lowest, and given that a sub does not require a stereo input nor fade or balance control, a bridged connection would have little effect on performance (bridging channels loses fade and balance control).
Another variable to consider is channel impedance. Every speaker is rated, in ohms, at certain impedance, and subsequently amplifiers are rated as well. At a minimum, each channel of the amplifier must meet or exceed the rated impedance of the speaker. A lower impedance means more power will be delivered to the speaker, but on the other hand if the amp is delivering too much power then both the speaker and amp could be damaged. Thus, if a speaker is rated at 4 ohms you should be using a channel that is at least 4 ohms or higher. With a higher impedance, less power is delivered to drive the speaker, but this will not damage or distortion. Of course, the way you wire in the amplifier and speakers can affect the nominal impedance as well, which is an entire subject on its own.
Finally, you need an amplifier that can provide the necessary capacity to all the speakers you want to hook up. A main issue with the OEM stereo system is that the factory amp is underwhelming (and frankly, so are the speakers) and unable to drive the speakers with enough oomph. Choosing an amp that meets or exceeds both the RMS and peak wattage values ensures each speaker has an interrupted flow regardless of volume setting. If a speaker is rated at 50-watts RMS and 125 peak, make sure the amp channel is capable of at least providing those two values, but realistically speaking, the amp should have a considerable margin over the bare minimum (50-75% more than what is needed).