If you like to work with your hands and have the time/money, I'd say build one. That is half the enjoyment of owning such a machine - to say you buillt it. Just depends where you are at right now in your life. If you think holding off a bit until things are more settled with the baby and you have your heart set on building, then I would wait and save your cash. To put this in your wife's perspective, she is mothering your child. Her needs and wants revolve around the baby. So anytime you can offer her say a break from the child, or ease a certain financial worry about being able to care for a child, the better her mental state and will score you points. In terms of getting her to accept your desires, you just need to tell her that it's important to you. Marriage is give and take, and if she approves of you taking the time and money from your child/budget to do this, then you should reciprocate and allow her the same. Pehaps she wants to take a "girls weekend getaway" with her friends? or buy some shoes! If you are willing to allow her the same, then any understanding companion will allow you to have yours. It's not like you are asking for some crazy XXX act or something. You're basically asking for money and time away from the family.
Now in regard to the timing, I must say I've been on the fence for about 10 years now regarding building my own cabinet. I recently got settled and financially able to finally do it. The state of availability of all the components is increasingly getting tight. CRTs are almost no longer being produced, Original leafs are hard to come by, other parts carry a premium, etc. While great strides have been made to reproduce parts or create new ones, the bottom line is if you want 100% authentic origial parts, it's quite difficult to make it happen today. So if that is the goal, I suggest you start sooner than later. If however you can accept an LCD monitor, microswitches and the like, then you can wait until time/money/wife permits.
My qualifications: We have twin girls and no family living nearby to "watch the kids".