re: I've been given $100K from an estate - Pay down mortgage, get new car, other?

Good problem to have. I'd make sure I was maxing all tax advantaged retirement accounts (ROTHs and 401ks for you and your spouse) for the foreseeable future. If you can't afford that on your regular salary, now you can, so set aside some cash now so that your regular budget isn't affected by the increase in retirement contributions.

I'd then set up college savings accounts for each kid with 20k each, and commit to putting another 5k in each per year. Leave it in index funds and re-position to more bonds as the kids get closer to college.

By now I've probably spent 60-70k, but I've taken care of college and strengthened (maxed) my retirement. With the rest, I'd set aside 10k for a nice vacation (or 3) with the family, and leave the rest in a money market account as an emergency fund. No stress in paying your low interest mortgage, no stress in investing for retirement, no stress in college, and cash in the bank. Don't go blow it on a car. Congrats.