I own a beautiful, 10 year old, PF85 and I am searching for a buy that will satisfy my needs for an add-on of "top quality tones." I know nothing about this subject and need some recommendations. I don't really require 500 tones that I won't use, I am obsessed with string tones. I've heard of groove boxes and samplers but all too costly for what I really want.
Since you claim to be new to all this, I'll risk the flames and recommended getting GPO. Its a compact, easy to use, and comprehensive library. It is also a nice starting point on your way to more advanced digital music production tools. If you jump now, its also on sale for 60% off ($139 dollars.) There's a good selection of strings (both solo and ensemble) to choose from as well as ensemble instruments with which you can build custom sized string ensembles. If you play live, this library is also designed to be played expressively live using velocity sensitivity for the note attacks and mod-wheel control for instrument timbre/volume, with sustain pedal control for instrument legato.
As I said, I think it'd be a great starting point for you. There are more comprehensive products out there, but for someone just starting out they may be a little daunting.