First demolding is taking the door molding off of your car it is advised that you do it on a hot day so that the glue is easier to come off. First what year is your car because apparently if it is a 05 and possibly 06 you may have some paint work to do after you take them off but if you are shaving your door handles then you will want to paint anyways. When it comes to headlights I would get halos and install them in the headlights before you get the ones off of eBay just my opinion some people might say they are ok but they are not that bright some of them. The tail lights I got were actually from eBay and they are a black chrome with LEDs and then when my car was painted the top sand side of the tail lights were painted to match. Now if you would like to put verticals doors on your car go through a company On the forums here there are a few vendors on here that sell them I think and they can walk ya through instructions on how to do that because I haven't installed them before.
For your exhaust what motor do you have? If you have a 3.5l v6 you can put on a used srt8 exhaust no problem and take out the resonators at the back and it sounds great. If you have a 5.7 or 6.1 then you might want to look at Corsa extreme I believe the model is or a magnaflow.
I hope this info helps..