Modulus is really only a measurement of stiffness. With that said, when it comes to fishing rods, you can achieve stiffness in a few different ways.
1. Use a higher modulus material that is naturally stiffer – Comparatively, IM8 graphite should be stiffer than IM6 graphite given that equal amounts of the material are compared.
2. Increasing blank diameter dramatically increases the level of stiffness
You can build a rod blank out of lower/intermediate modulus graphite (IM6 for example) and increase the blank diameter to achieve a level of stiffness equal to or greater than that of a blank made of high modulus material. With this taken into account, rods made of lower modulus graphite (to achieve X amount of stiffness) are usually heavier in weight than one built with a higher modulus material (with the same level of stiffness). Weight is a deterrent to performance (ie. tip speed and tip recovery). Conversely, rods made of lower modulus graphite have better elasticity and elongation properties and are less brittle than their higher modulus counterparts.
You can go the other way and make a slow rod out of a high modulus material by changing the blank taper.
As for the other question, “what should I look for?” Honestly, it’s whatever you can afford and feels good in your hands.