Thanks for taking a broad view of this, and asking the pertinent questions.
Here's the picture. We have about 30 PCs running XP Pro SP3 which use Cubase Studio 4 (education) as our main audio and midi environment.
We have standardised on the M-Audio Delta 2496 as the cheapest built-in soundcard that handled full duplex Audio In/Out well, and also MIDI In/Out. (Though Midi input is via USB these days, and we never send Midi out). The Deltas have excellent ASIO drivers, low latency. As you'll know, this card does not have a midi synthesiser built in. Two of our PCs have access to multi-input devices so that we can record multi-miked drum kits etc.
Cubase is used from Yr 7 right up to Yr 13 Edexcel A2 Music Tech.
The rest of the school's PCs are now running Win7 32-bit, but we didn't feel confident about switching OS at the same time. Testing our existing software on a Win 7 machine, we have found a problem running Cubase Studio 4, to do with getting available sounds (VST presets) to display.
We cannot afford to update our 30 Cubases at present, and we would not want to be running different versions of Cubase, so are sticking with XP for the moment.
Your latest Sibelius 7.1.2, able to run on 32-bit XP-Pro machines seemed like a good solution. It would have been daft to upgrade, this spring, from our 16 Sib 4 licences to Sib 6 when Sib 7 is more friendly to novice users, and was the same price upgrade.
Sib 4 was, and Sib 7 will be, used for composition and arranging by a few students and teachers, and for printing attractive looking scores after GCSE compositions have been prepared in Cubase.
The large Sibelius Sounds package could be fun, if we have disk space on each PC to install it, more memory and so on, and always remember to only use the "Lite" configuration. However, the price of any upgrade, or any proposal to acquire a 3rd party sound library, has to be multiplied by 30 to keep the PCs consistent. Hence the use of the free sfz player with various free soundfonts, and stuff that came bundled with Cubase: Halion One and a few virtual analog synths.
Our initial requirements are low latency playback so that live note entry works well, and reasonable GM orchestral playback for students who compose by the trial/error method.
One proper question I must ask... you seem to be hinting that unless we install several DVDs-worth of Sibelius Sounds to each of the 30 PCs, the basic Sib 7 program may not work. Is that true? If so, why?