Best advice is to keep it simple. Focus on brakes and tires first! Do some basic setup and then recognize while you are on track when you are at the limit of your basic setup, then go from there.

You already have SS brake lines, good! Fill them with good 600°F brake fluid (several were mentioned already). Do this every few events and be meticulous about doing it RIGHT and making sure there are NO LEAKS. Safety first! This and pads are your #1 safety item. Then an instructor, then good tires, helmet, seat belts, etc. Again, keep it simple.

Parts:

Use whatever rotors are on the car for now, and buy a set of these as backup whenever htey start to warp: Centric 120.40048CRY Cryostop Rotor
https://smile.amazon.com/gp/product/B003K1H2V6

Brake Pads, order a set of Hawk HP+ pads. You will get advice all over the board on what is best and blah blah blah. These are great bang for your buck and you can street them as well.
Hawk Performance HB361N.622 HP Plus Brake Pad
https://smile.amazon.com/gp/product/B000IEYNOM
Fronts are most important, do all 4 corners once you start to get comfortable with the car and/or go to a bigger track.

Think you're done with brakes? NOPE! Now let's talk heat. You're just starting to go on the track and want to be out for as long as possible to work on consistency and technique. If you're going to do any sizeable track with big braking zones you will want some way to keep the brakes cool. A lot of people poo-poo this step and then complain about how the stock brakes suck SO bad. I have had my s2k on CoTA, Road America, and BIR with zero brake fade with the setup I'm describing.
Option one: remove the dust shields, this will help reduce captivated heat
Option two: brake duct kit. DIY worked best for me:
-Lambert spindle mounts: http://www.lapponline.com/index.php?main_page=product_info&cPath=65_66_67&products_id=180
-Front Bumper inlets. WASP makes nice stuff but they SUCK to deal with and idk if they're even in business anymore. Several other options out there.
-Aircraftspruce.com has all the ducts and such you need:
Qty Ship B/O Item Unit Price Total Price
11 05-29910 SCAT-10 DUCTING 2 1/2" 7.500 82.50
6 QS200-40H BREEZE CLAMP 200-40H 1.880 11.28
1 01-00990 RESCUE TAPE 1" X 12' BLACK 9.350 9.35 (helps with rubbing)
3 05-02200 RUBBER "U" CHANNEL 1.350 4.05
50 11-04229 7" 50LB NATURAL CABLE TIES MS3367-1-9 0.040 2.00
50 11-13475 14" 120LB NATURAL CABLE TIES MS3367-3-9 0.150 7.50
50 11-04058 14" 50LB NATURAL CABLE TIES MS3367-2-9 0.090 4.50
Subtotal: USD 121.18


Ok great, now you have good brakes, you already said you have RS3 tires on there, buy a nice tire gauge and keep an eye on pressures & tire wear!
Joes Racing 32307
https://smile.amazon.com/gp/product/B00404WDUC

While you're at it, check the oil. Seriously, these things can (but don't necessarily) burn through a lot of oil depending on conditions and the track. Just keep an eye on it and plan for keeping an eye on it. Not a bad thing.


Get a good basic alignment on the car, max out the stock adjusters for camber front and rear, zero toe front and about .25" total toe in the rear, then you're good to go! If you are feeling DIY-ish pick up a Quick Trick alignment kit to bust this out quick and easy in your garage or at the track.


Once you get comfy with the car however you have it setup today suspension/wheel-wise, you can start to look at a few next-steps. Do all of these at the same time as its the combo that shines over any one part.
-lowering springs/coilovers (I did ohlins, so awesome)
-front swaybar (A simple eibach will do!)
-front bumpsteer correction kit/spacers
-17x8.5 or 17x9 square wheel setup with 255/40-17 tires. RS3's, Maxxis, VR1, are great lapping-day options. RE71R for quick/short/fast hot laps.

1 rule: HAVE FUN!