Overall most people tend to learn better this way rather than trying to learn all Excel formulas at the same time as this can be very confusing. Most of us tend to learn in smaller packages of information and you should look for a course of instruction that will allow you to do this. As you are learning to use Excel you should of course start by learning to use the formulas that you are actually using or are likely to have need of in the immediate future.
In terms of features, NeoOffice (donations accepted) is huge, truly a viable alternative to Microsoft office. It includes all the basic components: Writer, the Word processing component; Calc, for spreadsheets; Impress, the presentation module; draw, for the creation of graphics and diagrams; and Base, the database component. The spreadsheet component has some sophisticated features like Advanced DataPilot technology, Natural language formulas, an Intelligent Sum Button and a comprehensive range of advanced spreadsheet functions.