The decal paper is fine, it's the same stuff you are going to buy anywhere else, just in ALOT smaller sheets.
The software that comes with it, is pretty much worthless. I'm a horrible programmer and Icould probably do something better. MS Paint will work better then that, or download a copy of TheGiMP which is free and full featured.
The overcoat is just testors glosscoat. You can use almost anything. I like to use future, and spray it through the airbrush.
In short I wouldn't buy the kit, the extra cost is for the software which as I said is useless. I'd grab other software(either of the ones I mentioned above are fine but if you have photoshop use it). Then if you like the smaller sheets(hard to print with, but nice if you don't have large amounts of decals to print).
Use ytour favorite clearcoat to overcoat the decals a couple of times.
Before you clear let them dry quite awhile, where they won't be disturbed, or the ink can smudge. You need high quality print settings, but not too high, or it will deposit too much ink(ask me how I know with 3 ruined sheets), and they will look terrible, as it will tend to spatter.
That's about all I can add. I may do a tutorial on creating decals in the near future using photoshop. Just ther design process.