first multiplayer and then single player.
cause it isnt going to be a to small mod(il try t keep it as small as possible but..)
and there is a bigger need to multiplyer and single player.
and i dont now if you can make such a thing in singleplayer
on the hl engine.
i think that if you want sp then you should turn to games like il2 sturnovik or combat flightsimpulator 1,2,3.