Congradulations on the new Mustang!
In terms of upgrades, there are many Cold Air Intake Kits available online, and quite a few under $100. *Remeber that although it may not look as cool, a plastic cold Air intake will decrease transferred heat from the engine.
Secondly would be to modify the exhaust. A flowmaster true deul setup will not only inmprove performance, but will also give your Mustang a nice tone, and improve the looks.
Increase the spark in your car with upgraded coil, wires and plugs.
I do not have an sftermarket underdrive pully... but have heard positive reviews.
As for looks.... I like to keep it true to the Mustang spirit with upgreaded rims... either 35th Anniversary GT rims ( available used on ebay from others who chose to change them out ) or Bullitt rims are aggressive and cool looking too.
Mach1 Chin spoiler is a cheap and subtle exterior mod, as is the Mach1 grill delete kit.
Other then that.... it truly is up to you and what look you are going for. Enjoy!
