Hi Jeremiah,
If this is a D75-R then it should have a USB port on it. If this is an older SummaSign Pro D750 then it may have a serial port and one of the Keyspan RS232 serial to USB adapters from Tripplite should work well for getting the cutter to communicate with your computer. Just make sure that you get the adapter that includes the USB driver with it otherwise it won't function correctly.
Once you get the plotter to communicate you can download the free GoSign or Winplot cutting software and the only other software you would need from there would be a design program like CorelDraw or Adobe Illustrator.
We carry all the various parts and consumable items like blades, pens, etc. so if you need any of those items we have them in stock. We also carry some of the more unusual parts like motors, belts, etc. in case your cutter might need any of those items.
I hope the cutter works out well for you and feel free to contact me if I can be a resource in any way.
Best regards,
Phil Johnson
Airmark Corporation
(800)527-7778, ext 112
philj@airmark.com