Help with Midi

[David Sumbler]

When working on a project in Lilypond I usually set things up to
generate midi files as well as printable output, mainly for note-
checking.  But it is interesting to generate an ensemble midi file too.
I play these files with TiMidity, using the fluidr3_gm soundfont.

I know the (very) basics about midi instruments (although I'm a bit
vague about midi channels), and I know how to improve things by using
timidity to generate wav files which I can import into qtractor or
similar and then edit the combined result.

What I am about to ask is not really a Lilypond question at all (at
least, I don't think it is), but I am hoping that somebody on this list
can point me in the right direction.  Clearly I am not making the right
searches on line, because I have never managed to find the information
I want.

What I should like to know is
1) how can I use drum and other untuned percussion sounds?
2) I notice that fluidr3_gm has dozens - probably hundreds - of extra
sounds and instruments beyond the standard 128.  How can I access
these?
3) how can I use other soundfonts?  There seem to be a lot of free
soundfonts available on line, but I have no idea how I would use them. 
It would be nice to have a decent trumpet sound (the fluidr3-gm one is
very poor), the sound of violin section as will as solo violin, string
harmonics, timpani roll and so on.

Can anyone point me to a useful online source of such information?

David