Since this is something that is a vital component of your automobile, you don't want to be a cheapskate and buy poor quality parts, even though all you want to do is drop it when you park.
While you can use a 2-way system (front/back), you'll have body roll like crazy on curves. You'd do much better to go with a 4-way system. Some people will also tell you that you need 1/2" lines and valves. You DON'T. A 3/8" line size will be much easier to control and fine tune. A 1/4" line will offer even more control and not drop you like a rock.
That said, the only place I can say with 100% certainty that you'll get a quality system from is Air Ride Technologies. While they aren't cheap, the quailty of their stuff is top notch and the customer service is the BEST in the business.