The Motif rack mount unit will have all of Yamaha's best sounds on it but the price is about $1,800, Roland has a competing "Fantom" rack mount box and if you have a Mac then it comes with some string software instruments that you might just like, try those before spending any money.
If you need to buy good software string sounds then there are enough options that it will take some effort to evaluate them
This should get you started.http://www.sweetwater.com/store/search.php?s=software+string&go=Search
I'd call or email the place above. They have sales people who actually use the stuff they sell and don't just work on the phones. I've been able to ask advice on the "sound" of one product over the other and they can help.
Without knowing it would be hard to know it you's want "orchestral string" or "chamber strings" or "solo strings" But maybe you's be happy with a full orchestra sample library that has strings and woodwind and all. That's the trouble there are 100 to choose from
The target market for this is people with small or home recording studios who are music producers needing to add backing sounds to their recordings. So look at the outlets who sell to this group.