Sometimes, the simplest solutions are the best. Obviously, you want to get away from tape. The portable hard disk solution seems like a great way to go -- cheap and easy, and you can probably get 10 or 20 suitable drives for the cost of a decent tape drive nowadays, and avoid the cost of media.
You already have a "secondary server" in the form of the XP box used for your accounting system. You could easily connect the drives to this system, and then create a few backup scripts that move data files over from your existing servers over to this hard disk. You can use a simple tool like XXCOPY or ROBOCOPY to effect a synchronization, or you can pick a more powerful tool like Beyond Compare (under $50) from
http://www.scootersoftware.com/.
This doesn't cover your image backup issue, but perhaps there are other products that can likely be used in conjunction, such as Ghost for NetWare, or a Physical-to-Virtual migration tool that can create backup images of the NetWare box. Once you have the image, you can then copy it to an external HD pretty easily. (Consequently, this is also a side benefit of virtualizing the NetWare server -- you can not only move it to new hardware very easily, but you can keep running it in a VMWare environment for some time to come.) Of course, if you virtualize, then you can also use a VMWare backup solution, of which several options abound.