This thread should get you up to speed on the latest firmware, and you'll be able to flash any kitkit rom built off of NC2 or NC4.
To clear up any confusion, from what I've read and have come to understand is that NC2 is the leaked bootloader & kernel, while NC4 is the official. The NC2 bootloader allows you to flash backwards to MJE while the NC4 does not. The NC2 kernel is also unique with features I could try to explain, but barely understand at the moment. The other thing to know is that the NC2 kernel can be flashed over the NC4 bootloader (which is considered the unified firmware). That is what is needed for most of the kitkat roms.
If you don't care to go back to MJE, the quickest method to get up and running would be to flash the NC4 stock tar from here http://forum.xda-developers.com/show....php?t=2524572
, and follow the instructions in the previous post as rooting with TowelRoot takes literally seconds. The only thing you lose on the NC4 bootloader is the ability to flash backwards to MJE.
Now as for modems there are many threads around that talk about which one is best. Unfortunately this may depend on your device. Many people have reported very poor reception and download speeds on NC2/NC4 and have flashed the MJE modem reporting a fix or moderate fix. Others have reported signals and speeds better on NC4 than previous. Some think it has to do with NC2 bootloader being that it is unofficial, and some think it has to do with the release version of the phone as people with recent phones report great signal and speed. The other possible factor may have to do with Verizon's simultaneous upgrade to XLTE in some areas at the same time as the kitkat release. From my experience being on NC4 with NC2 kernel (by choice), I have no worse speeds and LTE in places I only used to get 3G. However, I have modded settings in Service Mode, so that may have effected things.
If you're considering roms after getting all set up check out these:
Jasmine Rom 1.8
Very clean, optimized stock rom with some S5 features. (2.0 is considered a beta by the Dev. He is wotking on 2.1. It is a full Galaxy S5 Ported rom.)
As close to AOSP as we will get without an unlocked bootloader. Excellent battery life.
Barebones, extremely debloated. Light, stable, add only what you use. (If you want any of the Samsung apps this is not for you as TouchWiz has been removed and many S apps require TW or Magazines to function.)