I don't think the orchestral playback synth supplied with Sibelius (Kontakt Player) allows you to fly in samples, you will need the full version of Kontakt for that. If you use a soundblaster for a soundcard, you can create you own soundfonts from your wave samples and use the free sfz soundfont player, and have Sibelius use that as a vst synth.
As for the pc, with any program that requires a lot of samples, you need lots of RAM, 1 gig or more with 2 gig preferable. Avoid Via mobo chipsets as these have proven problematic with soundblasters and soundcards, in general, although there are exceptions to the rule. Also avoid high powered video cards in your planned DAW as these vidcards have a nasty way of consuming cpu bandwidth leaving nothing for audio. Go to musicxp.net to optimize your OS for audio. And make sure you have a 2 drive system, not a partitioned drive but 2 physical drives. One for the system and programs, and the other for samples and audio. You can make the 2nd drive bootable as a just-in-case should the system drive suddenly fail. (I also have a 3rd drive for backup of important data - short of a catastrophic electrical anomaly in the pc, it's extremely rare that all drives will fail simultaneously.)