OK - I don't really know details for the pieces of software that you mention, other than Lotus support for Linux is not great (despite all the money IBM spend advertising Linux!). However, there is a great piece of software called VMWare which allows you to run a Windows 'virtual machine' from within Linux (you can boot Windows in a window on a Linux machine, and run native Windows software in that - I use it to run Office XP and Lotus Notes on my Linux PC at work). Highly recommended.
Virus protection isn't needed at present - there are proof-of-concept viruses that will infect Linux, but none of them are a problem in the wild. The Linux security model means that, assuming you are sensible, trojans are not an issue because general users don't have permission to do anything damaging to the machine. Security holes are more of a problem (as are trojans that exploit them) but are still rare for personal use - most holes are in software that is of professional interest, web servers and the like. These can be patched using YaST online update. The "out of the box" security settings seem very good in SuSE 9, although I haven't explored them in detail.
Finally, Linux is free - you're just paying SuSE for the cost of the CD and for customer support. You can use one set of disks to install on as many PCs as you like, although support options may be more limited. If you want customer support for multiple PCs they may ask for more money.