I’ve done this professionally for 60+ years.

MuseScore will do fine for this job. I especially like what they did to improve fonts in version 4.

MIDI is good, and you are correct, tons of MIDI files exist for jazz lead sheets. I prefer Music XML as the source if I can get it, because it has everything of MIDI (notes) and also all the text. MuseScore itself has a great site for downloading scores in either format.

Using a computer program for notation makes transposing for your horn players a snap. There will occasionally be some oddities to edit, like unneeded accidentals (E# instead of F, for example, but that’s not too common unless your key signature has many accidentals).

On your lead sheet, you can add chords but don’t count on getting chords in there automatically. So many online sources are wrong, or at least horribly simplified. MIDI doesn’t include chord info, but Music XML does. Unfortunately the XML standard isn’t fully implemented or standardized, so count on doing some editing.

I’ll answer specific questions as I have time.

BIAB 2025 Win Audiophile. Software: Studio One 7 Pro, Swam horns, Acoustica-7, Notion 6, Song Master Pro, Win 11 Home. Hardware: Intel i9, 32 Gb; Presonus 192 & Faderport 8, Royer 121, Slate VSX, Adam Sub8 & Neumann 120 monitors.