I agree with Cossie. Even the high end Coilover kits that fit the Z well do not use a true coilover in the rear.
Swifts+ koni sports, will give you a fantastic ride and the Konis will last forever.
KW v1's are also an excellent choice for what you want. They do offer a true type coil over in the front.
Either of those solutions will be ~1500 bucks and will be the last thing you ever need to buy.
The v1's will offer more range in adjustability and are an excellent product, the konis + swifts are just darn near perfect and will set the ride height just about where it should be(not super slammed though
)
Outside of those two offerings, I can't say the quality is there from anyone I have heard in that price range. You will hear nothing but happy people that went KW or swifts+ Konis.