After you go through and freshen up all the fluids it's kinda up to you... Good info in the Maint section.
What's more important to you? How much do you drive? How much do drive where the OME will be important?
A nice head unit will "feel good" every day and many will help you navigate. (Good threads in the audio section.) The HID's, to be done "right" should include an update to projector style headlights. (Search, there's some great threads on this...)
The OME WILL look cool and drive nicely but if you have a primarily suburban/urban commute I might prioritize the other mods ahead of the suspension. If you live in the country, use rough/dirt roads a lot and have a short commute I'd do the OME then the lights.
It just depends...:thumb3: