Hi Paulo,
One other way to get realistic MIDI files is to record them "live". There are a variety of ways to do this with guitar. You can buy a MIDI/acoustic/electric guitar such as Godin and Parker make, or get a MIDI pickup to put on an existing guitar (Fishman, Roland, and Axon make some fine ones) or on the less expensive end there is the Sonuus unit (which I have the first version which isn't so hot except on single notes, but I hear that they have improved tremendously). There is also a unit called the "You Rock Guitar", which is guitar-shaped, but uses what looks like fence wire for the strings, but is purported to allow a guitarist to do some pretty nifty MIDI work at a fairly minimal price point.
You can use any of these MIDI inputs with Notation Composer, and come out with a live sound AND sheet music. Composer keeps the actual recorded MIDI information, so all the little "humanized" factors are still there, while giving you a good clean piece of sheet music to go with it (provided you've stayed within some reasonable coherence with the metronome
If you didn't, you can use the ReBar feature to clean it up.)
Just another thought
ttfn,
Sherry