Quote: "
set resolution: 800x600 etc..."
SG Launcher
Quote: "set save/load system for a particular lavel"
Not sure which launcher, but I've seen one that does that.
Quote: "
set choice of sp or mp"
SG Launcher
Quote: "set difficulty (optional) (fpi script/marker triggered): number of enemies/size of the visual cone"
This can be done with various launchers, but you'd need to make three levels (with assorted enemies, weapon damage, etc.) per level (thus tripling the size of your executable). I still cannot remember the launcher where you can select levels, but you could have 3 unlock after each level (MyMap Easy, MyMap Medium, MyMap Hard).
To anyone who lives in England: I liked London when I went two weeks ago.