I am planning my car stereo and to keep it relatively cheap
but good I want to use 2 high quality 4" (infinity Kappa 42.5i) + the
basslink subwoofer.

Even though the 4" are small, infinity quotes their frequency range as
75 Hz - 21 kHZ (+3 - 3db)

SO with the subwoofer that can go to 120 Hz (adjustable crossover), I
should have it covered, right?

Any strong reason to add extra speakers (and spend extra) ? Would I
end up with poor midrange, despite what infinity says?

I am happy with the power handling on the 42.5i - 50W rms, 150 rms
peak, it is a lot for me.

Opinions?