I agree with Sardi, wipe the disk, do a clean install of mac os and PT10 and record to a external firewire drive, and you should be able to use that old imac. Be shure to apply all the optimization found here:
Computer Optimization Guides (Mac and Windows), and don't use the computer for anything else, and it should work with your mbox pro, even with as little as 4gb RAM.
You might also consider using an older version of mac os, I have PT10 running on a 2008 mac pro with a osx 10.6.8 partition and it's very stable. I don't think 10.8.5 was ever supported by PT10, though many people have it working.
Forget the SSD.
And don't expect to be able to use very many plugins.
The number of tracks shouldn't be much of a problem, as long as it's a reasonably fast external disk, no usb. That's where you should put your money.
BTW, if you decide to try this, I would advice you to learn how to make disk images of the internal disk to the external one, along the way. For this, you need to have the os installed on both drives. Not at all as complicated as it might sound,
much easier on mac than on windows. I'll give you a step-by-step if you want. The external drive would have to be, say 40-50gb +. I can also provide a os10.6.8 installer if you need it.
Go for it, and good luck!