It depends on your goals. Mods fall into 3-4 general categories. Straight line performance/power, handling, appearance, and sound. The factory performance is decent enough, so it would be difficult to get more than a minor bump in power without spending a lot of money. Handling is a great place to improve the Mustang, but there is a need to allocate the cost, since a good start would be $2-3,000, depending on the amount of work you do. One of the best ways to individualize a Mustang is appearance modifications. That is very subjective, but can be rewarding. Most mods can be done DIY, and in an open order. If you just want to go for better sound, then a CAI and axle backs are your options. Beware that most muffler systems can have an annoying drone over long drives.
2013 MCA Edition-Sterling Grey
Roush body kit - Splitters/hood scoop
Whiteline suspension w/Koni shocks