Short answer. Don't give up your Windows laptop. "Numbers" is Apple's spreadsheet "equivalent" to Excel. Unfortunately, even the version available for MacOS devices is not nearly as functional for professional use as Excel. And the version for the iPad is an even more limited version. There are Excel emulators (Documents to Go, Quick Office HD, and most recently CloudOn) but none provides equivalent functionality for complex formulas or, in some cases, even fairly common functions.
There are persistent rumors that Microsoft is developing a version of its Office suite for iOS devices. However, those remain rumors and even if they're true, the software is unlikely to be available until sometime later this year.. I'm guessing that the best case scenario is sometime in the second or third quarter of the year. In addition, like "Numbers," Excel on the iPad is likely to lack features that are available in the Windows version of the software.
None of this is to say that the iPad with one of the software packages cited above will NOT meet your requirements. But before giving up your laptop, I'd recommend you test the iPad rather extensively. By the way, you may also want to look into whether the web pages you use in your business support Safari. A number of more specialized web-based applications limit their support to Internet Explorer or currently require support for Flash.
