If you already have a fast notebook, then you're halfway there. If you don't have a notebook, I will suggest just going for hardware modules since your interest in solely in piano. Getting a notebook just for piano sound is not worth the money.
Besides the notebook (which must be quite fast with lots of RAM), you also need an audio interface and a midi interface. A standalone piano module may cost anythig between 100-400USD, depending on which one you get. So it may not be cheap. The benefit vs cost increases if you get more libraries.
Since the CP300 is your first digital piano, I will suggest not jumping so fast into upgrading your sound by purchasing new hardware. You must have gotten it only recently since CP300 is very new. Spend more time with it first. If it is too bright, play around with the EQ. Try to layer sounds. In other words, you should be VERY familiar with all the sounds that come with the CP300 and see if you can get a sound you find most useful. Since the CP300 is not a synth, the amount of editing of the sound you can do is very limited. But at least you have EQ and effects to toy around with - it cannot do much since you can't alter the orginal waveform, but you should be able to mellow the sound if brightness is a problem.