Not an expert, but I like many of the Cubase Pro synths. Foremost among these is HALion, but it’s hard to get decent sound design out of it without going to the full HALion, which is sold separately. (And it’s quite impressive.) I also like HSO (HALion Symphonic Orchestra), also sold separately and also a great value. The strings are fine, and would probably be OK for what you have in mind. My main problem with HSO is lack of reverb control. Honorable mention goes out to retrologue and dark planet. Dark planet (DP) is good for subtle atmospherics and additional layers, but not something I’d use as a foreground or lead instrument. I’m pretty happy with the groove agent stuff that comes with Cubase, but I’m easy to please in that department.
People with more experience than me would likely recommend Massive, or something like that. However, I like to keep my universe of learning curves as small as possible without being confining, and I don’t want to mess with too many vendors, so I like to stay close to Steinberg. Lacking broad experience with other stuff is a reason not to take my opinion too seriously. I guess people will say it depends on exactly what you’re trying to do.
I’d get into sound design If I had a long life ahead of me, but it’s more practical for me to deal with it when it comes up. Selecting patches is sort-of like getting into sound design. It is tedious and time consuming to preview hundreds of candidates. Now I’m considering a 2-step process of finding a decent patch to open with, then tweaking it into what I need. Full HALion is the way to do it. Unfortunately, sound design is a discipline of its own, with certain listening skills and raw materials. It’s not easy.
[Designing / Scouting for Sounds]
As I see it, the deal with sound design is that you’re designing an instrument, and that instrument has to be expressive if you want the player to be able to move the audience. Therefore, it has to respond to changes in velocity, if nothing else, and the response can’t be too abrupt. I say “All funny sounds sound like funny sounds”. They don’t sound like there’s a person behind them, so they have no soul. They are artistic distractions. They are sound effects. There’s no connection possible. Now, that’s an overstatement. Organs and harpsichords can work quite well when the composer has a strong vision. Note that they don’t come with a lot of reverb, multi-delay, distortion, or other effects that would suck up the lion’s share of the arrangement space. So, one must ask how a designed sound is to be used. To communicate an elaborate vision? To communicate human feeling? Some kind of blend of those two? Making the music big (composer’s vision) seems to favor clean, interesting sound design. Making the performer big (soulful connection) seems to favor responsive sound design. Making a funny sound big doesn’t work after the novelty wears off. It’s like a movies without a great story or great acting – just have 2 hours of explosions and gun fights. You never want to see again, even though you likely forget what happened soon after seeing it the first time.