Turbos will always best a supercharger in both horsepower and torque at equal boost levels even if both are tuned correctly. A turbo compressor is much more efficiant at processing air than any available supercharger including centrifugal and roots / srew type blowers.
However, turbo kits tend to be a bit more costly especialy with your model mustang which turbo kits normally require a tubular K member. Labor is much more extensive and as a result much more expensive if your not planning on doing the work your self.
If your power goal is very high, and if your motor is capable of handling high power levels, than the twin kit or a large single setup would be wanted. Remember, with turbo kits come a lot of little luxeries such as an in car variable boost controller. With super chargers boost adjusting comes by way of pulley changing. Of coarse your gonna have to be out of the car and this takes about 20-30 minutes. However with an in car boost controller, you can simply dial in more or less boost, in your car while its running!
So if your setup isnt quite ready for high boost applications, you can run it at 6 psi and when your setup is ready, you can simply dial in more boost.
However larger turbo setups tends to spool up a little later in the RPM's than smaller ones. If your looking for immediate boost, turbos can still best the screw / roots type blower by way of a smaller turbo. With the big loww-end torque of the V8, a smaller turbo will fully spool up just off idle.
As far as mods to your motor for turbo use: it would basicly be the same as supercharger / nitrous apps for the internals, intercooling is highly reccomended with turbo apps and of coarse as with any setup, tuning is the key to long life and performance.
Check out this web site, its a bit out dated but it will generally throw you in the right direction http://www.turbomustangs.com/redirects/kits.htm