Think of it this way -- the basic technology of all cutters was developed back in the late 1950's by Hewlett Packard, and the plotting programming is still a variation on HPGL.
The developer of SignBlazer (Jerry Bonham) was a programming wizard and knew how to incorporate all sorts of drivers for cutters into his little sign-making application -- all based on the HPGL standard (or DMPL).
Although your cutter is "old" it still functions EXACTLY like any newer one you will purchase.
The only advantage of getting a standby unit is to avoid waiting two or three days for a replacement to arrive in case yours decides to fail. Since these things are mechanically fairly simple and robust, failure is not something you would really expect (unless a motherboard craps out or something electronic of that nature).
If I was in your position, I would be setting aside a bit of cash every week from all that work, in a "Fund" that can build up over time until you have an amount sufficient to go grab a Graphtec.