I like the Eibach Springs, but I think you are better off spending the money on a matching Kit. That's springs, Struts, and sway bars. I have read a lot of good with the Eibach, but there are several good ones out there. I personally have the Roush Kit on mine.. after all its a Roush Mustang so that is what it came with. I am not saying its the best, but one of you options. Suspension is another one of those things you have to Judge. Is Handling you priority? Is ride comfort? You can get the best of both worlds with a complete Kit, but with just adding springs, you may not have as good a ride quality with the lowering. If you have a $1500 budget you need to think about where you are going with the car, and how much of the work you can do yourself... after all. A $900 suspension is just that, unless you have to pay to install it and then add the price of an alignment, suddenly you may be in the $1200-1300 range.
$300 Lowering Springs
If this is all you want to do with the car:
That's $1500, but would be a good jump in looks, power, and handling. If you can install it yourself, throw in a Cold Air Intake with the tuner.
If this is just a first step in Mods... I would go with:
1. Full Suspension: $700-$2000 depending on what you want out of it
2. Exhaust- $500 just cause it sounds like a mustang should
3. Tuner and CAI- $600
4. Rims/ Tires- Hard to put a price because there are so many different choices but figure $1500-$2000 for a good set of rims with tires
5. Gears- not sure what you have but 3.73 will really wake up the way the car feels if you need more $500- $600 installed?
6. Brakes- Stock is great but if you get far enough in mods you need to have better brakes to stop the power $2000 plus install
7. Supercharger... big power gains! $6000-$7000 plus install
Its easy to hit thousands in mods quickly! First come up with a plan on where you want your car to go.
2011 Roush Stage 2 Convertible Race Red with painted gloss black stripes, Charcoal Interior-Roush # 11-0306
Bama Tuned, Roush Front and Rear Brake Kit, Roush Exhaust with Square tips, 20" wheels, Roush seating with Red stitching
Roush Billet Grille matching lower 3 piece, Matte Black side and top stripes, White Face Gauge cluster, Convertible Light Bar Purchased 27Dec2010
Dyno'd 401 HP/ 390 Ft lbs at the wheels