What softsynth would you recommend for realistic strings, woodwinds and brass etc?

None.

Omnisphere has some semi-realistic string sounds that can work at times.

You need sample libraries if you want strings, woodwinds and brass etc.

Curious - you were hired to compose cinematic classical music for a film, but have nothing in the way of orchestral sample libraries? Isn't that sort of like me getting hired as a chauffeur without owning a car?