My two cents:

HIDs first, then spoiler, then angel eyes.

HIDs are a functional mod which you will be grateful for every single time you drive your car at night.

Spoiler will help "complete" the outer look of the car.

Angel eyes, while nice, are a bit of a pain to install (or expensive to get someone else to do), and IMO are more of a "finishing touch" and less pressing than the preceding.

After those three, I'd start thinking about the hood/grill etc.