Wow was not expecting a car audio chat here lol.
A few key buying factors for you using head unit power: note if you are not interested in learning any of the theory. Don't. Simply know it's important.
1. "Coaxial speakers" are what you want. They have a woofer/mid driver and a tweeter. Don't get too excited about gimmicky 3- four ways, .. ext. A simple appearing speaker with a little Dome Tweeter in the center, will do a fine job of accurately reproducing your favorite artist noise.
2/3. Rms power/ sensitivity these two speaker ratings coincide and work against each other. generally the more power they can handle the lower the sensitivity, and vice versa. IT IS EXTREMELY IMPORTANT TO NOTE THE alleged ratings of these numbers in no way can be used to quantify anything. Every one test their own speakers and basically make variables that should not be in there testing so they can boast great numbers. think of speaker manufacturers like engine manufacturers that can claim any torque and horsepower curves they please. It's just how it is especially on the middle to low end price range. There is a standard test the selling company will list ( some acronym I forget) "... compliant - yes/no" anything that says yes you can compare apples to apples.
Power: simply how much power you can give a speaker. Using head unit power you are uninterested in high power handling, and it will work against you.
Sensitivity: if I remember correctly this is supposed to be how many db's (how loud) it is 3ft away with 1 watt of power.
For anything that's compliant 90+ sensitivity is really good.
Key design aspects of good sensitivity/ of course also representing low power handling what is not important to you are;
Small "voice coil" 3/4 -1, higher resonant frequency/simple/light weight build design of moving parts.
all of that translates to pretty much getting louder/(still accurate) music out of a limited power source.
4. Impedance: every speaker has an impedance rating, and it's extremely important. unlike the above they are generally accurate within a window.
I don't really know it's worth a check but I would bet with confidence your stock speakers are 4 Ohms rated. This would get more long-winded than it already has if I explained it but the important thing to note is;
More ohms = less loud/amp preservation
Less ohms = more loud/amp runs hot, and can burn up.
2 ohms is at the point where your stock head unit may burn up/ cut out going into thermal protect.
3 ohms is about the happy spot. some speakers are manufactured to be 3 ohms. I strongly suggest purchasing speakers that advertise "reduced impedance" or 3 ohms.
4. Speaker baffles they are foam baskets that isolate the back of the speaker. it's gives you the effect of a sealed enclosure, helps protect the speaker from overextending itself, will give you more accurate bass response, and they cost around $10.
Put it this way, I would rather have $20 Pioneers in front of a baffle, then hundred twenty dollar Alpine's in free door air.
That's the best 2 cents I can give. Hi-5 feel free to tear this apart but, it's the best I could do for getting to the point.