My 2 pence worth, buying a frame and building to your own spec as VW said can be costly.
It will be cheaper to buy RTR.
Then may be up grade/change parts as and when you are ready.
I'm trying to go tubeless at the moment, 4.5"X26" tubes weigh in at nearly 500grams each.
Now on my second attempt.
First attempt worked , but only lasted 48 hours and 1 ride.
Now looking in to using inner tubes cut down the middle to seal tire to rim.
Best bike I've ever purchased my fatty is.
Just love to get out and ride it. Puts a BIG smile on my face
The comments from passing riders and people is real fun.
Hope you can get what you want, and most of all enjoy it as much as possible.
Any more thinking of joining the fat bike gang!!!
Trev