If this car is used for AutoX with only 5% concern given to street use, I would skip the TA springs and go straight to these:
MM Weld-In Rear Spring Adapters, 2-1/2 [MMCA-1] : Maximum Motorsports, the Latemodel Mustang Performance Suspension Leader!
The installation instructions are here:
These are spring adapters that weld onto the center of the stock upper rear spring perch. They allow you to use a 2.5" diameter coilover spring in the rear. This will give you a huge range of spring choices to work with. The only downside is that since you would be using a linear rate spring with them, the spring will come loose at full droop. This will occur when the car is jacked up for instance or when going through a driveway at a 45 degree angle. Since the spring perch has a several inch long pilot tube in the middle of it, the spring has virtually zero chance of ever falling out.
The rear spring rate you use will be a function of the car weight, tire size and compound. A heavier car with larger, stickier tires, will need the highest spring rate to control the chassis lean angles. Start with that information.
The only reason to use a conventionally mounted spring in the front is due to cost. A coilover will give you better ride quality and handling due to less friction in the suspension, less overall weight, and the ability to adjust ride height and corner weights. The only time we recommend the front Super Race spring is when there are class rules that don't allow coilovers. Customers who switch over from SR front springs to coilovers always comment on how the ride and grip improved, even though the body roll/brake dive didn't become worse.
The front coilover spring rate is a function of the items mentioned for the rear spring rate, plus the rear spring rate itself.
If you install coilovers in the front, the only required added cost is that of the coilover kit. The spring price is about the same. If I remember correctly, the Steeda k-member allows use of the stock FCAs, so there is no required added cost there. However, in the future you have the option of losing about 14lbs of unsprung mass and/or extending the wheelbase for better weight distribution.
Once the front and rear spring rates are chosen, then the struts and shocks are picked which have adequate damping for the spring rates and car mass.
BTW, your TA link has an extra http\\ that is keeping it from working.