Depending on where you live you can probably get a nice used bike, especially if you have some mechanical background tuning your own bikes. It's an interesting dilemma...don't want to spend too much in case it's not used, but if it is a crappy bike it will not get used much. If it was me, I'd look for something I could convert to a cargo bike. With a kid, you may find yourself hauling lots of stuff in addition to the kid.
Whatever your wife decides in terms of future use, you would have an option and a decent cargo bike.