I have a 91 Mustang with 130,000+ miles. The motor is stock with no upgraded internals or bolt on upgrades. I installed a Powerdyne 9psi supercharger a few months ago and have had no problems. The car runs strong and makes 270rwhp/322tq.
I didn't get it dynoed before the supercharger but I'm sure it was probably in the 180rwhp or less range. Even for Powerdyne I gained a significant amount of power and I love it.
I probably spent around $2,500 on the supercharger, fuel mods, chip, and tuning. I also installed everything my self.
The reason I decided to bolt on a supercharger is because I didn't think it would make since to only upgrade the heads and cam on an engine with 130,000 miles on it. The engine would need a total upgrade and rebuild which would be very expensive and since my engine was still running strong it would be unnecessary. I'm going to just keep the engine stock for now and get as many more miles as I can out of it then upgrade/rebuild the engine once it goes.
If you decide to go with anything other than Powerdyne, 9psi will probably give you around 320-350rwhp with increased cost.