Brakes are an interesting subject. You can get the stock brakes to perform, it just take the right pads, fluid and cooling ducts. Depending on how long you plan to abuse the brakes, for example 10 minutes, 20, 30, an hour at a time will dictate how far you need to go. If it's an occasional blast down a curvy road the right pads and brake fluid are enough. If you do more and are cheap add cooling ducts. If the fluid still boils it's time for larger calipers and rotors in addition to appropriate pads and fluid. I like slotted myself (355 mm) but be aware, if your really using your brakes hard the front rotors won't last and the calipers will need to be rebuilt often. I have Stoptech's BBK on the front. I'm on my 3rd set of rotors and have rebuilt the calipers twice. In other words, the initial investment isn't the final solution. Find and get the setup with the cheapest consumables. Even with a BBK and huge rotors you can overheat the front brakes, it just takes a longer duration of abuse and super sticky tires...Hoosiers for example. I haven't used every street tire available but from what I've had, Yokohama AD08's were real good unless overheated at which point they fell off a bit. The Kumho RS3's were what I used most often for sizing and price to performance. Both have been super ceded but I imagine the RS4 and new Yodo's are similar. Then I stepped up to Hoosiers. I was racing with 245's on the front and 275's on the rear. My rear brake calipers are stock with slotted stock sized rotors and they've done the job and never needed attention. You don't need to change them for any reason other than looks.
Don't lower your car too much. Crossfires have limited shock travel and if lowered you could end up ridding the bump stops. It won't matter much on the front (new bump stops are cheap) but the rear is another story. Run out of rear travel suddenly in a corner and the rear gets snap loose. (no warning). Another problem with lowering the rear a lot is the axel boots tend to tear. Lowering is good .... go too low and you'll be sorry. Spring rates ....... the best overall for a street warrior is what K&W sells. They are 630 front and ..... I believe 450 rear. I have to double check on the rear but it's close to the stock SRT rate so you don't need to change anything except if you want to lower it a bit. For racing I preferred the K&W's with stiff bump stops to Ground Control, if I'm being honest. The only advantage to the Ground Control is the ease of adjustment.
If I have more thoughts I'll add them later, if you have questions....if I have answers......

Les