As my internal diskspace is not sufficient to host my entire music I use a small 2.5 external USB-Harddisk
with 500GB to connect to the USB-Hub (see Laptop-where?), enough space to do a Safety Copy
of the entire laptop..
Speakers to connect to the soundcard of your PC begin literally at 5 Euros and end where your budget ends - you can easily spend several thousend Euros.
Of course everybody defines "enough power" in another way, but spending 100 Euros should already get you a quite decent system,
compared to the standard carsystems usually installed.
My former Creative Inspire - System was a 2.1 - System (Left, Right, Subwoofer) - this keeps the satellite speakers small
while having the necessary pressure from below - consuming less than 3,6A (12V).
Former Onboard system,
Satellite rear view...
The Laptop is used to transfer the mp3´s from a playlist to an SD Card,
to listen to the music you just connect a portable MP3 Player. Without consuming your batteries for the lapotp....
MP3 Player with SD-Card-Slot, SD-Card see picture above...
Look for a "SD Card Reader with MP3 functionality", these are the reasonablest players.
You´ll buy them for 15 Euros at the Onlie Discounter of your choice.