1. No, it's not wrong to tell an employer that you will be back when you're not 100% sure that you will be. What IS wrong is our country's policies around maternity/paternity leaves and the fact that many parents have to return work when their children are 6 or 12 weeks old. What might be better is for you to say that you are planning to return to work, but that you do realize you're entering an entirely new phase of your life.
2. Like Jules (?) said, you and your spouse are so far ahead of the game in retirement, savings, etc. (stop w/the car loans, though--at $180K/year, you should figure out a better way) than many people! You are in MUCH better shape than many mothers or fathers who stop outside work to become SAHP.
3. Tracking expenses is certainly important, if only because you'll may realize that many of your expenses are discretionary. I found those very easy to cut back on when I quit full-time work to freelance after my first child was born. People like to talk about the expenses of a child, but I myself found that all the little self-indulgent habits I had completely dropped off after I became a parent. I used to spend money on clothes, going out to restaurants w/friends, getting my hair cut at a super-nice salon. I still did these on occasion after becoming a parent, but a lot less (these cuts were painless).
4. Taking care of others' children as practice is totally exhausting and not necessarily predictive of how you'll feel as a SAHP. (I know because I took care of nieces as an aunt before I became a mom.) Doing so is a good exercise, but it really isn't the same as taking care of your own. It's like being a substitute teacher vs. having your own classroom--very different experiences.
5. Be the best employee you can be before you go on maternity leave. If you do decide to stay home, stay connected to people in your field however you can (consulting, small projects, even dropping by your old office for lunch w/coworkers every now and again). If you can keep one foot in the pool, returning in a few years (or even more) will be so much easier.
Good luck, whatever you decide. These things do have a way of working themselves out.