I've been around playing various releases for, oh, 5 years or so now, but never created a forum account. Since I've tried the game vanilla and spent a good bit of time fooling around with the .ufos as well, I thought I'd give you some advice...
1: You don't need a special program to unzip the .pk3 files. They are ZIP. If you change their name to give them a .zip extension, you'll be able to extract them. MAKE A COPY OF THE ORIGINAL FILE, but give it a different name. I use 7z, so to make sure the game doesn't try to get the original files I call it ufos-original.7z
2: (wait, what do you mean "make sure the game doesn't try to get the original files"
) - glad you asked. If you simply extract the files, say, to base/ufos/* -- which is where you should put them (that means, create the directory) but leave the original pk3 file where it is, you're modifications will be read (or perhaps ignored? not sure of the parse order or precedence), and immediately overwritten by the original configuration. That means, you have to either hide the originals (by renaming the ufo.pk3 file) or deleting it (not recommended - you may want to revert to the original configuration at some point if/when you make some change that crashed your game...)
3: To increase your starting money, after you'd got your ufo files sorted out right, open campaign.ufo in a text editor (not Word!!! use a text editor or developer IDE. I use Sublime Text 2, but TextPad is a good one, NotePad++ etc) then, either pick a difficultly level or copy and paste one - make sure to change it's name - then find the line for "credits" and change the number. Keep in mind, the max money you can have is 10,000,000 credits.
You can go on an edit all sorts of other things, but once you've made your armor impenetrable, gave all your guys Superhuman (100+) stats, make your interceptors fly as quantum speeds, invulnerable and insta-killers, made your guns shoot (effectively) infinite ammo at 1 TU per shot across the whole map with no spread... (etc etc etc), just keep in mind, it's fun at first for sure to one-hit every alien with reaction fire from 5000 yards... but the game isn't finished, so it gets boring VERY fast when the battles aren't really a challenge.
