I also have a new 2007 V6. I am in the same boat as you. Everything I did was pretty much under $2,000. I bidded and shopped wisely on ebay. I did things one step at a time.
18 inch konig crosshairs rims from
www.wheelsdirectonline.com. I got them for $1,200 shipped and mounted with free tires. Free shipping too if you can wait 3-6 weeks. 225 tires up front, bigger in the back (agressive stance)
Chrome upper and lower grill $40
GT suspension kit for $43 on ebay! Includes GT front and rear sway bars, GT shocks and struts. Here is a vid about making a V6 better than a GT:
Project V6: The Story of the New Take Off Mustang - Stang TV Forums
Adjustable Panhard bar and camber bolts for $50.
Eibach pro-kit lowering springs $71
Chrome CCFL Halo lights, chrome front bumper, and clear back bumber lights $150.
Chrome tail light bezels $10
JBA single axleback $80
C&L Intake $92
Diablosport Predator ($263) They will write you free tunes, just post on their forums.
Pre cut Mustang window tint $25
Mustang 10" white stripes $35
Brushed aluminum dask kit (cheap sticker kind, but okay if you just want to cover the radio) $15
Stainless Steel Ebay headers $90
Secret is to bid last 7 seconds on ebay....
The very first thing you should do is just get a JBA or Borla single axle-back, they sound great. Don't worry about duals. Then get a prederator tuner. Actually get that first! Because the tuner has stock airbox tunes that are fun to use.. Then get the intake I guess and have diablo sport write you some tunes.