I can't comment with any personal experience when it comes to MSD coil packs w/ wires or the screamin demon coil packs and wires as I've never used them. I haven't heard anything negative about either one if that helps. Might want to do a search and see what some of the other guys with 96-98 GT's are using.
I can comment on the plugs though. Motorcraft, NGK, and Autolite are some well-known brands that many guys like to use in these 4.6ltr Mustangs. I use Motorcraft plugs with no problems and my brother is running a set of Autolite's with no problems either. I recommend going with a set of copper spark plugs if what you have in mind is performance. Platinum is good for mileage, but not so great for performance. You can go the iridium route, which is good for both performance and mileage, however, they are very expensive. Which ever you get, gap them at .054 for the best n/a performance. You might want to also run a step or two colder than stock. I'm running the Motorcraft AWSFA-12C's, which are two steps colder than stock with no problems.