You should be able to use the keyboard, mouse, and monitor from the Dell with a Mac mini (I'm assuming the Dell has an external display, keyboard and mouse - of course if it is a Dell laptop then I have assumed wrong). At most you would need the correct Mac mini to monitor video adapter (mini-Display port to VGA, or mini-Display port to DVI, or mini-Display port to HDMI).
Of course if you want a new keyboard monitor and mouse (or better yet Magic Trackpad), then an iMac is a nice machine. I own a 27" and my Mom has a 21.5". Both are very nice systems.
The Cisco router should be fine, and you should be able to attach the 500GB Seagate to the Mac. Although you may want to reformat the 500GB Seagate external disk to use a GUID partition style, and format it with an HFS+ file system. The reason for using GUID partitions is then you can use something like SuperDuper or Carbon Copy Cloner to turn the Seagate into a bootable backup for the Mac. If you do get around to doing this, you would use Applications -> Utilities -> Disk Utility to put a GUID partition map and HFS+ file system on the Seagate when the time comes.
NOTE: If you get a Mac mini and then decide later to replace it with a different Mac, the Mac mini can be used without a monitor as a home media (or backup) server for the other systems in your home. I use a Mac mini with lots of external storage as a home backup server, using CrashPlan to backup the other Macs in the family.
