I'd say to do it right you'd need :
the wiring harness from the 2.0T along with all the computers
the trans from the ST (sure, a stick shift 5 speed would work, but c'mon...)
suspension from the ST ( sure, it would run fine on a N/A suspension, but c'mon...)
The ST has a lot of different stuff in it than just the turbo. It definitely would NOT be a plug-n-play swap.
Older, non computerized cars are easy. Take a 89 S10 truck and drop whatever carbureted engine you want into it. Beef up the other components, and bam- you're off to the moon. Honestly, for the time, effort, hospital bills, and costs of equipment, and quarters in the swear jar you'd probably be able to buy a new GT mustang and smoke the an ST anyways.
If you are looking for a "sleeper", It would be so much more easy and cost-effective to buy an ST and stick Titanium front/ rear bumpers and skirts and spoilers on it.