As far as stats, Heath and Musou are important, as it would require both the Heath and Musou upgrade to get anywhere near 999, and thats with only tanky characters. Speed is a big one too (I modded mine around 400 or so) it helps to zoom through missions. Atk and Def are not really needed with a good weapon and good abilities, but still, that 3 out of 5 skills that are needed, and since they are all stored side by side, might as well be able to mod them all. Just my 2 cents.