Here is my software set up for open source music making (I often use midi and audio input):

Core software:

• Ardour as my DAW.
• Liquid SFZ as my SFZ player.
• SurgeXT as my soft synth.

For SFZ instruments:

• Virtual Playing Orchesta (free full orchestra)
• Versilion freeware instruments (mostly orchestra)
• Blonde Bop drumkit

Raw samples:

• samples.kb6.de for drum machine samples

For effects:

• LSP plugins for my basics (eq, compression, etc)
• Airwindows Plugins for fun effects (and some basics)

Ardour is supposed to have a midi related update for its next big release, so stay tuned.

From my perspective, the simplest set up would be Ardour and SFZ instruments. Mainly because I'm quite used to those two.

For set up you'd just open Ardour, make a new midi track, place liquidsfz as the first plugin on the track, then open liquidsfz and browse to the SFZ file you want to use.

Then you just draw in the midi notes you want using either the edit or draw tool.