170k gross household income. The withdrawl wouldn't need to be anything immediate - just something I could withdraw from with a few weeks notice.
Okay, I'll give you a recommendation and you and others can critique it from there.
I think the best bet would be to go with a Roth IRA. This means that as long as your gross income is less than $181k/year you can contribution up to $5500 per year for you AND your wife (so $11,000 yearly). With this account you are free to invest in the stock market - broadly speaking. You can put it in a low expense ratio stock index, let it grow, and be on your merry way.
With a Roth IRA, your contributions are not tax-deductible. If you withdraw from it early (before age 59.5), there is a penalty ON GAINS ONLY (not contributions) of 10% plus you must pay taxes (your current tax bracket) on those gains as well. Therefore you are fine contributing $5,000 and then if you need it, can withdraw that $5,000 without penalty. But be prepared to pay a hefty sum when it grows and you take out the money prematurely.
Also noteworthy the order of withdrawal is this:
1. Regular contributions - always tax- and penalty-free
2. Conversion contributions (doesn't apply to you) - which come out on a first-in, first-out basis
3. Earnings on contributions
So if you contribute $5000, it makes $1000 and you withdraw $5000, no taxes/penalties. If the same scenario you were to withdraw all $6000, you'd pay 10% penalty on the $1000 gained and also have to pay taxes with respect to your tax bracket on that $1000.
The best part: When you are 59.5 you are able to withdraw from it and everything is completely tax-free. Also, there are no mandatory distributions meaning you can let it grow until you die.