I don't think Notepad will do what you want. The short answer, from their
comparison chart (click on Composing and Arranging Tools) is that Notepad supports transposition By key sig. for entire piece only. (There's also a link to a pdf with all the gory details, though it's not as clear on this particular point.)
In other words, it does not permit different key signatures per staff, and you would not only have to know how many steps to transpose (isn't that much obvious, or am I missing something?), but you would also have to express that by selecting a key signature, rather than number of steps or by somehow selecting an instrument name.
Still, with a cheat sheet, and if you were willing to transpose once for each instrument and then print their part, it might be workable, and it is free. It also seems to do a decent job of importing from midi files, though it doesn't seem to give you any real control over the process.
Notepad can be really frustrating. I tried it out a year ago when I was looking for transcription software, and ended up buying Finale Allegro - found it on sale for $150, and it had enough features to satisfy me. But I haven't used it for your purpose, so i can't speak to that offhand.
