Well, what you do from here is dependent on how much money/time you want to spend.
The basic problem is that the Mach460-equipped Mustangs had almost completely different audio wiring/speakers from the normal Mustangs.
I converted my base factory audio system (my Mustang came with a nonfunctional POS aftermarket head unit/CD player) to the Mach460 system. I can post up a detailed walkthrough if you want; it'll take about $250-400 worth of parts off of eBay (or a salvage yard) and you'll basically strip your car interior to rewire the audio system (takes about 2-3 hours if you know what you're doing).
Or, you can buy an aftermarket head unit ($ varies wildly depending on quality) and a wiring adapter ($20 maybe?) and use the basic audio system existing and resell the Mach460 head unit/CD player.
Or, you can buy a factory nonMach460 head unit on eBay, use all the existing wiring, and resell the Mach460 head unit/CD player.
The Mach460 unit WILL NOT work with the base audio system (at least, not without some fancy custom wiring, and it'd probably sound like garbage afterwards).
Let me know if you need that walkthrough - it's long.