I use exactly the same two deject mentioned, Sony Acid Pro and Fruity Loops Studio, plus Reason and sometimes Ableton Live - I have pretty old versions of all them that were given away on magazines to entice you to buy the hideously expensive newer versions. All of those are more music
production tools rather than music
creation, at least how I use them. I feed them a MIDI file, then add instruments and effects to make them sound how I want. Almost all
the stuff I did on acidplanet was in Acid Pro with sounds from Reason (just from MIDI files other people had made plus loops), but you can get loads of free VST plugin instruments which are easier to get working in FL Studio, so I tend to use that more now.
For actually creating music how you describe you really only need a basic MIDI editor such as Anvil Studio, while I find input easier with my DX21 as a MIDI input device, you can just click to add the notes yourself. I can't read music well enough to do that though and go with what sounds good, but if you're comfortable with staves it's probably the way to go. It does has the option to input with guitar tabs (chords) or a piano roll (notes as letters like a sequencer) if they're more familiar to you. Then you can put the output into one of the aforementioned tools.
Other options are loop/beat generators which pretty much mimic what drum machines like the 808 did, right down to the interface. They are basically step sequencers where you can enter notes as patterns for each bar, then order the bars into a song. The programs at the top have these built-in usually, but I've found getting them to play alongside an imported midi track quite tricky - so I tend to go for one creation method and stick with it throughout. Since the sounds can be anything from a MIDI note-trigger, a drum beat, a loop, a note sample or even a full loop sample you can build up a complex track fairly easily.
By loops I don't necessarily mean pre-set sound clip loops, but loops of note patterns that you've programmed yourself - the key difference is that you're not making a mashup of WAV or MP3 files, but using each 'loop' or sample as a note from an instrument that you might play yourself on a sampling keyboard.
I grew up with actual 4-track tape recorders, real 808 drum machines and synth-sequencers like the Casio CZ-101, so the way the programs work come fairly naturally to me, but it can seem a bit daunting at first, and the way they are all slightly different doesn't help, but there are plenty of video guides around.
Other tools I've used are cgMusic - which will randomly generate MIDI based on seeds and keys you put in, which is a good starting point if you're short on ideas, a lot of
my stuff on last.fm started that way.
Another useful tool is Band-in-a-Box, which I got an old copy of from my Dad when he upgraded. He uses it to put in chord sequences to accompany himself when playing the saxophone, but I put in a simple MIDI tune then apply various music styles to it. It basically works out chord sequences and embellishments which go with your tune, that you can then edit into your MIDI file for depth. I use it quite a lot when remixing C64 tunes and other stuff I've already got the MIDI of.
Some tips on the MIDI files would be to put each instrument in its own track or channel, or even to save each bit of the song in separate files, certainly save each revision as a new file - it's easy to copy-pasta and glue it all together in the production phase, but the devil's own job trying to split it apart once it's merged. Also take a note of the tempo in beats-per-minute, so that if you want to add loops or other elements from elsewhere later on, you can pre-edit them to the correct tempo so they stay in time.
Like most things, it better to not get too complicated. My
favourite thing I've made is a very simple tune (from Phantasy Star 4) and I've been working on and off for ages on a new version of
my most popular tune (a C64 remix) which at the moment is so overcomplicated that you can scarcely hear the melody, but sounds so much better when I break it down and let the simple bits of it be heard - but I haven't been able to do it justice yet.