1) Springs:
Most springs are pretty decent as long as you get them through a reputable vendor. The super cheap Ebay specials are as bad as they are inexpensive. Stick to the name brands and you will be fine. H&R make a very popular spring for your car. I believe they state a 1.3" drop. Anything over 1.5" and you should really consider rear camber arms. The skunk2 springs give a 2" drop which I think looks quite good but is very low.
(H&R, Eibach, Skunk2, etc..)
If you lower the car on stock struts with a set of springs you will wear the struts quicker. They may blow sooner and especially with a large drop so keep that in mind. Coilovers are popular because they include a strut but, if you haven't the budget all at once, get good springs and pick up a set of TruHart struts from redshift motorsports as your budget allows. Or run the stock struts until you run into problems and then upgrade.
TruHart struts:
http://store.redshiftmotorsports.com/product-p/truth-h505.htm
When you lower the car the front will not gain any negative camber. If it was 0 degrees it should stay 0 in the front. More front camber means better handling and more grip in the front.
In the rear, lowering will give you more negative camber. This is a very subjective number. I would not recommend anyone run more than -1.5 degrees front or rear on a street driven car. If you lower the car 1.5" in the rear or more your camber will be well over -2 degrees. It is true that camber does not wear the tires as badly as a poor toe setting but excessive camber will wear your tires. -2 or more is where you will start to notice wear on the inner shoulder much quicker. That said, I think -1 to -1.5 in the front and -1-ish in the rear is a nice balance of performance and tire life.
Rear camber arms can be picked up quite cheaply on Amazon as there is an SPC set rebranded as Dormans for less than $100 usually. That is a steal compared to the actual SPC arms or Skunk2 arms at near $300 for a set.
2) Camber bolts: Front 2012+ SI needs the 16mm bolt. SPC Part #
SPC81280
For the front just pick up a set of SPC camber bolts. One set in the upper strut mounting hole should give you -1 to -1.3max adjustability. I have been running -1.2 in the front for a number of years on my Civic. I also run -0.8 in the rear but I did run into a rubbing issue so you may need a bit more depending on tire/wheel/low. Get two sets if you want more than -1.3-ish. Id' start with just one set and get the max you can out of them with both sides being even. On my car we were able to get -1.5 on one side but only -1.2 on the other so we set them both the same.
http://store.redshiftmotorsports.com/product-p/spc81280.htm
3) Strut Tower Brace:
Strut tower braces are popular for all the wrong reasons. On older cars they did help a lot with handling and chassis flex in the engine bay. Cars that have the struts out in the center of the bay can be kind of flexy from all the engine weight and handling forces running through there. Tying the tops of the struts together will help alleviate those issues.
However, on the Civic the struts are jammed up against the firewall and the A-pillar comes down to join in that area as well. It is one of the stiffest areas on the entire car and needs no extra bracing. If you want one just for fun and the look, get one. But know that it will not improve your handling. The plastic cowl covers that area pretty well so even the bling factor is kind of moot. There are better places on the car to spend money if you want it to handle better. If you want an easy bolt on project and like the look, get one. My thought is don't waste the money.
4) What to buy first:
What have you got so far? I recommend everyone the larger rear sway bar and flash pro. Those two things with a set of good tires and you'll have a totally different feeling and driving car. Get some front camber bolts if you're on a budget and max them out for some added bite.
Short list:
Wheels/better tires (expensive)
Larger rear sway bar - Just get the Progress 22mm bar. It's instant performance you can feel and use. I think the 24mm bar is a little overkill and the 22mm balances the car nicely without getting you into trouble at highway speeds.
Flashpro - If you don't have this get it.