I think it is easier to just buy the kit, you might be able to save a few bucks buying everything separate, I'm not really sure, but i figure buying the kit saves you all the hassle and time of trying to find all the hardware and hoping they have the right bolts at the hardware store and blah blah blah...
As far as you building custom bumpers... You should have to worry whether you buy the kit or not, if the kit comes with brackets to raise your bumper, so what, don't use them, no biggy. You're still building your bumpers in anticipation of the body being 2" higher, regardless of whether of not you bought a kit or got ebay pucks, (the bumper won't know the difference
)
Did I answer your second part right? Not sure if I understood it entirely.
What are fab four bumpers? I have never heard of them?