I find Apple's Mail app to be quite sufficient for my needs.
I prefer MS Office for Mac, for maximum compatibility with Windows users. If you don't want MS Office for some reason, there are many alternatives, such as iWork, NeoOffice, LibreOffice, OpenOffice, Google Docs, etc. Searching the forum will reveal dozens of threads discussing these various alternatives and the pros and cons of each, such as this small sampling:
Try Mellel, it's supposedly written with various (multi-lingual) scripts in mind. Their home page isn't exactly fabulous so check their support forums if you have specific questions. It isn't free but not that expensive either (equivalent of $40, from the AppStore or their own).
LibreOffice is free. While the non-native interface and the default shortcuts can be clunky at times it seems arabic script is supported (check the last answer). Also, it uses OpenDocument format, which is nice in this proprietary world we live in. That's actually the reason we use it at work (archiving among other things, which needs longevity and all that).
Can't really tell you about other advanced features as my word processor nowadays is a text editor+xetex.
