^I'm no firewall expert, but I like firestarter, and the documentation and Web site are good:
Continuing with the suggestions for SL Business Editiion, a couple more things that should be included in my opinion are the e-mail applications Evolution and Thunderbird. Evolution is the Open Source alternative to Microsoft's Outlook on Windows, but does not have a Windows equivalent as far as I know, whereas Thunderbird -- an Open Source alternative to Outlook Express -- does. Business users will want something that is cross-platform, hence my suggestion for Thunderbird. Also, as it's cross-platform, it can be set up to share e-mail folders and files with Windows on a dual boot PC.
I'd also recommend including KAlarm with SL BE. It's easy to use and works well, and business users may find a good alarm program useful.