Originally Posted by jalami

1) The file system doesn't matter, you'd access them as Windows shares, which is available in Mac OS X.
2) I use Office 2008 for Mac, which covers Word, Excel etc. As for instant messaging on multiple networks I run Skype and AdiumX (ICQ/MSN/Yahoo/AIM). Mail.app (the OS X mail application) could replace Outlook Express depending on how you use it. Firefox is available, and so are some good FTP apps such as Transmit. Quicken Mac 2007 is available as well as QuickBooks Pro 2007 for Mac. In any event I would suggest making sure you get a laptop with a good amount of RAM (2GB+) so that you can run VMware Fusion with Windows XP to run Mac apps alongside any Windows apps that do not have Mac versions.
3) Yes. The only exception is the wacky Office 2007 files (.docx, .xlsx) for which Microsoft offers a conversion tool to download.