If you are going to be online anyway, and hopefully with decent bandwidth, you could try CloudOn or OnLive. These provide you with access to server based versions of Microsoft Office. The interface is less than ideal, and depending on server load and your bandwidth it may be a bit slow, but it provides the closest thing to 100% compatibility with MS Office that you can get on an iPad. You'll also need a DropBox or Box account to copy files to/from the service.
If you don't have to actually use MS Office, Numbers is a good app. Probably the most full featured spreadsheet app you can get on the iPad. It does a good job of importing and exporting MS Office files; but is not the best choice if you are going to constantly move the same file back and forth between Excel and Numbers. Each conversion of the document can cause errors, which will probably accumulate each time it gets converted.
There are other third party MS Compatible apps that have decent MS Office compatibility, but are more limited in features. Since they don't convert the file (they just ignore features they can't edit or display) the chance of errors accumulating is less. The current darling of the tech media is hopTo. I have no experience with it, but you might find it worth checking out.
Also often mentioned, and worth comparing features:
QuickOffice
Office HD
Documents to Go