Rangers had regular and premium sound system options
regular has the amp in the head unit
premium has a head unit and separate amp
So obviously if yours has head/amp in same unit and you want head and separate amp then you will need to change the wiring a bit, and of course get the separate amp when you get the head unit.
Good link here to wiring for Ranger sound systems by year: Ford Ranger Radio Wiring Diagram
There is alot of misconceptions about audio wattage, stereo makers post theoretical watts, 20watts per channel in reality would be 4 to 7 watts, and that is really enough inside a truck or car.
Usually complaint in car or home systems is a lack of power, not loudness, but power, power is lower frequencies, and smaller speakers simply can not generate longer waves(lower frequencies).
Adding a powered subwoofer, for a car/truck 8" is fine, 10" if you have the room, will give you that power.
For homes a 12" powered sub will really improve the sound quality for music and TV.
Wattage is a 10 times progression, 40watts is not twice as loud as 20watts, 200watts will be twice as loud as 20watts, so if you have a 20watt stereo upgrading to a 60watt would be a waste of money if loudness is what you are after, it will be louder but not as loud as you might think.
Also be aware of a speaker's wattage rating, you will end up damaging the speaker and amplifier if you change amp wattage to near speaker wattage max.
Speakers themselves will have a rating on how efficient they are, SPL is often used, 90dB @ 1W/1m is an average speaker, this means if you input 1watt of power to speaker you will read 90db(audio level) at 3ft away(1 meter), a good speaker may have an SPL of 96dB @ 1W/1m, a poor speaker 84dB @ 1W/1m.
So upgrading your speakers to a better SPL will increase volume