Try to help, hopefully others can chime in too.
1) You can install yourself, there is a DIY by FLXR8 -
https://www.infinitiscene.com/search/1/?q=9333-2003-2008-FX35-FX45-BC-Coilover-Install-DIY
Much easier IMO than H&R/Eibach install due to no need of a spring compressor.
2) You can do the math yourself, but if you do it right and replace all the suspension parts (struts, bearings, rubber bushings, rubber mounts, shocks, H&R's/Eibach's) - it's not that much more for coilovers.
3) 60-100k is the life of the KYB struts and shocks, they can last longer but performance will degrade if they haven't leaked/failed yet.
4) My strut bearings last only 30,000 miles, and I either have to live with a "clunk" over bumps or replace them every 30,000 miles. This is just my experience. The metal strut bearings went up in price a lot, the plastic ones are very cheap. Labor is time consuming.
5) When you adjust the height of the car, you will want to get an alignment. Firestone around me provides $140 for lifetime alignments for as long as I own the car. My local mechanic does a laser alignment for $70 as a baseline.
6) Coilovers can be customized for you -- the main springs, you pick your spring rate. higher the spring rate, the stiffer it is. Shocks are valved to match the strength of the spring to dampen the recoil.
7) Shocks for both front and rear are 40 way adjustable, to go from hard to soft.
8) Front is no problem to adjust, there is enough control and even an extra camber plate if you do slam (which you said you won't).
9) Rear should be able to get within factory specs if you lower no more than 1.3" than OEM. If you go lower, you will need a camber kit OR if you have a good mechanic, he can easily slot your subframe to get your alignment into factory specs.
Does that help?
Hey guys, after installing my new wheels I want to lower it but not to the point to where i cant go over speed bumps. I have looked a the H&R's but from what I have read the coil-overs are the only way to go. My ride has 54k on it and I know it is only a matter of time before struts need to be replaced. I realize the coil-overs are adjustable in regards to height and stiffness but I have a few concerns that maybe you guys can help me with. Once installed i realize that they have to be dialed in and Im concerned that it will be a long tedious process to get them the way i want them, also will the car have to be realigned and will I need a camber kit to prevent tire wear? Also, will i be able to adjust the stiffness to still get a nice soft ride? I don't want to slam to the extreme just to make my wheels fill the wheel-wells and i don't want hemorrhoids in the process, I already have a horse and don't want my fx to ride like one. I know I have asked alot and I am ready to purchase but I am a total idiot when it comes to suspension, I am very mechanically inclined and have no reservations in regards to the install and adjust myself and don't like paying other people to do what I can do myself. Any help would be appreciated. Thanks in advance!