I know a few VSTs for cellos and brasses. Check out East West/Quantum Leap’s Symphonic Orchestra or Edirol’s Orchestal. The latter is the most easy one to work with, but EWQL’s just sounds alot better.
Cello solo: Garritan Gofriller Cello
Violin solo: Garritan Stradivari Violin
Trumpet solo: Samplemodeling The Trumpet
Entire orchestra: East West Symphonic Orchestra or the cheaper Garritan Personal Orchestra. Avoid Edirol.