For editing is used usually Cubase, Nuendo or Protools. This didn't solve your dilemma though because even when you buy this software you don't have much to start with. You would need to get some sound banks. To get you started, search for some free soundfonts which will get you running. For them you need some VST soundfont player. (google is your friend there)
If you want to go more pro, the cheapest way (not sounding very good though) is to go with Edirol HQ orchestra, which is very easy to use orchestra VST plugin (about 120MB) . For more advanced you can grab Sedliaczek orchestra or Garritan personal orchestra (google is your friend there). For the best results you can use VSL, which is veeeery expensive.For synths I am using Distorted reality package which is pretty good.
Typos everywhere...