I think combining different free orchestral samples/plugins is the key.
Two free plugins i like is the 4Front Upright Piano and DSK Overture. The piano is pretty excellent for a small plugin like that, best free piano i’ve tried at least. The orchestra has some very nice qualities, i feel the quality varies a bit between the instruments, but for free it’s pretty good.
ZynAddSubFX is only a synth, but it does some pretty darn good sounding strings and lots of other stuff.
I have also used Edirol Orchestral VST a bit, but stopped using it because i’m not shure if i had the rights for it. It’s a discontinued plugin though and i downloaded it from a page that said it was free, but i’m not shure if it is really legit.