Basically, at this point, you're out of luck. There is no Excel app available on the iPad (I've tried them all) that has the depth of what you need for your requirements. My own special need is for drop-down cell validation... a seemingly simple function that greatly enhances what you can do crafting spreadsheets for small screens, but alas, not a single app has this.
I've heard through the grapevine that MS does talk about developing Excel for iPad, as they already have a Mac Office component. But, the issue is always do they want to sell more devices that directly compete with their own current/future products.
Bottom line right now is all the Excel apps are anaemic to the point of being useless to any Excel power user.