In my opinion extra horsepower, no matter how much/little, is always worth it. The cold air will get you a few more horses for now, and if you do more serious mods in the future you will see bigger gains, and you will be happy you got one. As for the type of cold air however, I would stray from K&N. They make a good product but are severely overpriced. I'm sure they want about $300 for a plastic tube and a filter:no If you are gonna buy a kit I would go with JLT's RAI or CAI, I think they are the best "bang for your buck". Do some research on Ford Mustang Cold Air Intake Kits, Mustang Intakes at AmericanMuscle.com - Free Shipping! they have a few different styles/prices to choose from as well as customer reviews and images.
i've got a drop in K&N I'll sell you for $15 plus shipping. it's in good shape. honestly i've had a CAI but now IMO the only CAI I need is my procharger CAI's really don't do much more than a K&N is going to do.