Who said go for subs too - a nice 60Rms amp for the front speakers will make a huge difference to the sound quality. A car that can do 150mph will be a nicer drive at 70 than one with a top speed of 90
I'd rather run an amp at 40% of maximum power than a headunit internal amp at 80% of maximum power .
Small amp won't cost much more than 80 quid, set of components can be picked up from 60 quid up .... wiring kit no more than 20 and a couple of hours time to fit
One of
these
which is 4channel - 2 spare channels for rear speakers or bridged for a small sub later.
One pair of
these
in the front doors.
One of
these to wire the amp in.