You should change the shocks with the coils.the shocks for the air springs don't provide enough damping for the coils I feel the ride quality is diminished. I have installed many coil conversions from Atlantic British. The fit of the parts is excellent. The height of the truck after conversion is slightly lower than standard height and the springs tend to sag within a year.I have recently installed a Terrafirma kit in my personal Rangie and have found the ride to be excellent.I used the 223hd kit .The lift is approx. 2 inches from standard height. The rear springs need trim packers with this kit as the rear ends up approx. 1/2 inch lower than the front if you don't have a winch.Check Alan Bates website at Rockrover.com for a more detailed look at tech. info on coil conversion.
OME is the smart way to go, more money up front but no sagging in the long run. Do your research! There is so much info on this site about coils. Alan Bates site, is also good resource as already stated..