Attenuators/Attenuverters are nice to have. Some CV inputs don't have pots built in to them so while you can plug a modulation source into them, you don't have a direct way to control the amount of CV.
The Maths module can handle attenuverting for you. Channels 2 and 3 are dedicated towards that if you take the direct output of 2 or 3 at the bottom of Maths. You can also buy dedicated modules for attenuverting such as the Befaco Attenuverter that offer more features (like offset). The Maths can replicate this function, via the "sum" output, but it's convoluted using the Maths.
I think this rack is good to go as far as exploration. I would get in some time with the rack before adding anything else. What to add next depends on the direction you want to go in.