As you can see, the costs are heavily dependent on the hard drive, and that included only a single 3TB drive.
I would do as suggested and determine the feasibility of using your existing system as the basis and adding 2-3 hard drives to it, since they are the largest cost of any system.
As suggested I'd test your existing system with an unRAID USB stick to see if it boots (remember to unplug all your hard drives before trying so you don't hose them).
You could then transplant the hardware into a better case if you wanted to.