I'd concur with Jerry. There are enough differences between Numbers and Excel, that learning one isn't adequate preparation for using the other.
The open source applications mentioned by Jerry are modelled more closely on MS Excel, but even these have differences in the features they support.
MS Office Home and Student (Mac) would appear to be the best choice in your situation.
Another big advantage to having Excel on your home system is that you may some day need or want to work on a business file at home. You will find that compatibility issues will frustrate you to a degree well beyond the cost of the Office suite. iWork is not good for collaboration with Office users except in the case of the most basic types of documents.