commuter build questions

I have an early 90's trek 700 multi track hybrid that broke behind the
bottom bracket last year, so I had it welded and it now it is cracking
again. I really like that the bottom bracket is really high in this
frame which helps with clearance when I go off road.
I am thinking about using a 29er frame, I want to reuse as many parts
as possible, 700c wheels etc. I know I will have to go to 1 1/8 fork,
headset etc. unless there is a shim system that is reliable. I ride all
weather ( snow) and bike does get abused. I ride with a rack and
panniers and fenders.
Any reason not to get a 29er frame?,
I think the more reasonable option is the nashbar X frame.
I also might spend the extra $85 for the carbon fork. Is this a bad
idea?, it is 1 1/5 lbs lighter than the steel fork.