Hi Conan
You can install either the black and white graphics pack or the colour pack - it's a personal preference really. Hagar has done a fantastic job of recolouring tech images and leader files, so it's worth checking out just for the wow factor. You can actually install just the 0.49 CORE file which includes basic graphics, but the black and white/colour packs really add to the experience.
The lack of balance means that Teghettoff modified the basic CORE mod from Armageddon so it would work with AoD but it wasn't polished to include all of the new features that are in Aod eg. Artillery, new trade and economics etc. so you may find some countries are overpowered, while others are underpowered. You will still get a pretty solid game, but some strange results may occur.
With regard to your download - once you install the CORE file as normal into your AoD directory, it should then copy the CORE files into a separate directory, leaving your original AoD game intact. You can then run the game with the shortcut that the installer creates on your desktop. This is called moddir and means that the mod is separate from the original game. You can then play either without any issues/crashes occurring.
We are here to help, so keep the questions coming if you are unsure about anything.
EDIT - Just remembered that 1.05 had issues with CORE save games not reloading due to the moddir setup, you can probably try running an earlier version of AoD - 1.04? or alternatively try the 1.06beta which is available in the AoD Tech Support forum.
Hope this helps,
El Alamein