Hello Emjay,
Take it from me personally, don't use any carbon fibre wrap, used it, took bulk shit from people, then pulled it all off. Took some advice from the people here.... The "Clean" look is the way to go and "cheap" never works either. I agree as it makes the car easier if/when you come to sell. I did however end buying a genuine carbon fibre bonnet scoop and side vents for my car from JP Carbon (I've adjusted the link to suit your model)
http://jpcarbon.com/Impreza-2004-200...aspx?sid=24176
I also kept the original parts so you have the choice to revert back if you feel the need, or sell the aftermarket parts for some extra cash if you sell the car.
As for the front lip, i would definitely go with abs plastic, or any other plastic first before carbon fibre as it will crack after being repeatedly smashed/scratched against the floor (not saying you wouldn't be careful though)
Also gauges are a good choice
Defi are very expensive, but i have never heard anything bad about them
If you are just after the look and are not relying on accurate readings you could always go generic or use a cheaper brand, also for your model of car there is a few triple gauge docks on eBay for the centre console (where the clock is)
Personally i would drop the vortex generator though, just my opinion but i don't think they look to flash.