With Mac OS X Lion you would have to run MS Office 2008 or 2011 if you want to go that route, or iWork for Pages...while you can save Pages in Word format, be aware that not all functionality is transferable...things like tables to don transfer from one to the other. If you need full Word and Excel conversion, then go with MS Office 2011 as you are guaranteed the right end product.
For presentations, however, you really can't beat Keynote...heads and shoulders above PowerPoint.
Neither Apple's Pages '09 v4.3 or latest Numbers applications are completely compatible with Word and Excel respectively. Yes, these Apple applications can import Word and Excel content that are supported, and can export to .doc and .xls file formats. However, if you are regularly exchanging files with Microsoft Office users, you will have much better compatibility with MS Office for Mac 2011.
You will need OS X 10.7.5 installed to purchase and download the latest Pages and Numbers from the Apple AppStore.
You could also look at LibreOffice 4.0 which may serve your needs as well -- for free.
I think you mean word processing and spreadsheets.
Pages and Numbers do both very well. Pages actually has full spreadsheets within the word processing documents, so you can combine the two, along with superb charting and excellent DTP.
Just be aware that these are not Microsoft Office clones, they both work and produce documents that are excellent in their own right doing things that neither Word nor Excelcan do but also not doing everything that the 2 Microsoft products can do.