One of the problems with using MIDI for instruments such as trumpets is that it isn't good enough simply to trigger a note and let its do its thing, because a human player will vary the loudness and other qualities of the note while it is playing. So in order to have anything like a realistic-sounding trumpet, you have to have a way of controlling the notes while they're playing.
One product that addresses that issue explicitly is Garritan Personal Orchestra
. Let me suggest that you check out the demo at the top of their page as one example of what's possible with MIDI.