FIRST- I won't say if carbon is worth the extra cost, but more importantly: If you haven't spent three hours or so on a road bike you may hate the radically different and aggressive position. I'm assuming the old Trek MTB has you sitting upright more than almost any performance road bike will position you. Fit is everything. Like a nice Italian suit, if it doesn't fit... not really worth the cost.
SECOND- Include your wife in this frivolous purchase of yet another bicycle. You don't want this to become a "bone of contention". She should consider the cost of the most mundane cardiac surgery.
THIRD- This being your first road bike you need to consider this as a learning experience. You may want something else (probably more expensive) sooner than you could imagine.