What would you say is the max required Agility to be able to hit everything 90% of the time?

A level 50 warrior needs an agility of 145 to get offense up enough for a 90% chance to hit (not counting equipment bonuses, etc). FWIW at the end of my first game, my level 53 warrior had an agility of 150 (112 without any equipment, or the bonus from a certain item from Verdistis).

I don't have an exact number, but it seems to be around 100. Using Heaven's Blessing to boost my 140ish Agility to 190ish, it was at 94% both before and after, so that seems to be a cap. It did, however, significantly help with defense, the enemy hit rate dropping from 33% to 19%

Yes, the maximum chance to hit is 94% and the minimum chance to be hit is 19%.

differences between classes (minor spoilers)

Early Starter Tips (NON-Spoiler Version) (contains much of the information in the above topic)

Thanks all. :)