buy all of them, they all work well together and will be more versatile. There is another volume of strings for x-sample. This other volume has the strings that are struck and plucked, plus some fx effets.
Start out with the Dan Dean collection. It is very good, and the sounds are versatile with most music. When you have the additional funds, start to buy the three x-sample libraries. Begin with vol. 5, this set has violin, viola, cello, & bass. This is the set that has pizz,bartok pizz, spiccato(sounds like a very crisp marcato)& fx. Volunes 10 & 11 have non-vibrato sustained samples, tremolo, harmonics, and more strange stuff. I had forgotten about the cello vibrato in the x-sample series. This is a good stero cello to have, but this can wait until much later. I would even suggest buying at some point the Larry Seyer\'s Upright Acoustic bass. This one is primarily all pizz, but it comes in handy. I think is was made for jazz, but you can use this instrument for other stuff as well.
I bought most of these libraries one at a time when I could.