There aren't many files you need to edit the GTA mission.ini file. In fact Notepad is the main requirement (though you may want a revised version with better search and replace type stuff, I use Notepad+). But there are a few files you should get:

Mission Template Description. This is the file written in house at DMA, basically a list of each command you can use in the mission.ini and a definition of what it does. A couple of errors in it, but overall it's the most valuable resource you'll use whilst scripting.

Mission.ini Guru Site. A long time ago a guy called Henrik (I believe he wrote the missions for Burger City) created a site called Mission.ini Guru. Unfortunately that site is now dead, but he offered it as a download for offline browsing so I've put it here (without his permission) as another resource for you lot. Basically it's an HTML version of the MTD, but it does include some added information on how to use the commands. I use it rather than the MTD simply because of the hyperlinks.. saves you searching for commands.

The files below contain the mission.ini and english.fxt files for use alongside the 7 Day Guide. You don't need to use them, but you may find it useful to have the complete mission scripts for reference.
Day 1 - Day 2 - Day 3 - Day 4 - Day 5 - Day 6 - Day 7