Oh boy, jumping into Vienna as your first sound library is not really too intelligent of an idea...
First of all, the VSL samples ARE extremely high-quality, but they compensate by requiring an extremely good computer (or multiple ones, as is generally the case) to run them efficiently. Also keep in mind that VSL samples are all recorded completely dry, so you're going to need to invest in a good reverb plugin to be able to make anything resembling a realistic mock-up with them - they do NOT sound as brilliant as the demos lead you to believe right out of the box.
Your ensemble concern is understandable but not really a big issue because, as you said, there are expansions to give that functionality if you don't feel like building the ensembles yourself out of the individual patches.
Either way, as a composer who is just starting out, I can't bring myself to endorse a purchase of a VSL library. You simply won't be able to get the most out of it, and the whole process of writing for these samples and, more precisely, the post-production work involved may very well turn you off the art entirely.
If you want to start out as a composer and venture into sample libraries for realistic mock-ups, you can't go wrong with any of the EWQL Symphonic Orchestra products. The price is on par with this package, only it includes far more ensembles and instruments and articulations, is less computer resource intensive, and comes with natural reverb recorded right in the samples so you don't have to worry too much about your mixing right off the bat.
So, to summarize, if you want to be smart about this purchase, my recommendation is that you start and end your searching somewhere around here:
EastWest/Quantum Leap Symphonic Orchestra Platinum Plus Complete :: East West/Quantum Leap :: Developers :: Sounds Online
Good luck.