I love my Fatbike and am a roadie as well. I can say, while everybody else is right that a plus bike is faster, lighter, etc. etc. etc., I never bought my fatbike expecting these. That being said, I have just recieved my new carbon chiner fatbike rims and am in the process of building them up for the Road Race on Route 66, and the wheelset will be coming in at under 2300 grams with an SP Dynamo front hub and a DT Swiss 350 rear. That is plenty light enough for me. I enjoy riding the Fat due to it absorbing much of the road vibration which makes my hands go numb after 200 or so mile days back to back to back.
As far as a build kit goes:
Brakes: Sram Guide series are great. Easy to bleed, easy on and off, and plenty of stopping power. They also are not ridiculously expensive.
Drive Train: I am changing mine from the current Sram X7/X9 to the XX01, 1x11, next month. Doing this for several reasonsons: 1. I find that having the front deraillure and extra chain ring adds too many variables and I question if my gearing is right, just getting rid of it. 2. By dropping the deraillure, chainring (and harware associated), shifter and cable, I am loosing close to a 600g. 3. It leaves one less thing to go wrong.
Wheelset: Nextie 65mm Black Eagle II with SP Dynamo front and DT Swiss 350 rear. I picked these both for reliability. I like the dynamo because it allows me to have always on lights for the road, I am able to charge batteries, and lets face it, I don't have to charge my phone when I get home. Rear DT Swiss, for cost and reliability, nothing is much better.
Frame: Currently riding a KHS 4 season 3000, however, researching some other (carbon) options. Most fat frames have about a 69 degree head tube angle and being a roadie, I like mine a bit steeper. I have seen some at 71 and will probably go with that. A steeper less laid back seat tube angle is also a look/feel I like.
My 2 cents. I hope it helps a bit.