Thanks for the detailed advice, Marius! I improvise on the piano, and I have a Yamaha Clavinova, which does MIDI, but it's old - some of the keys are going bad, and it has a floppy disk drive... but more importantly, I've never learned how to create MIDI files by improvising and then edit them to create better compositions.
I've created electronic pieces of music that I edit and process (adding reverb and other effect, cutting and pasting, etc.) using Audacity. This is lots of fun, but I don't have the tight control over rhythm that I'm hoping I'd get using MIDI - I'm just editing sound files directly.
I've got Windows so it sounds like I should get Cubase. I like electronic sounds as long as I have enough control over them so I should start with those and try Native Instruments after I get serious! That sounds exciting but I need to work my way into this.
I will answer your math questions separately when I'm feeling more awake. I took a second dose of a shingles vaccine and it's really knocked me out all day. (My wife too, otherwise I wouldn't believe it's the vaccine!)