Don't bother using nitrogen in your tires to keep up tire pressure. Air is about 80% nitrogen anyway, and air or nitrogen will leak out of tires over time. Just check your tire pressure regularly. It can make a substantial difference in your gas mileage if your tire as under or over-inflated.
If you're serious about buying a car, ask the dealer to let you borrow or rent a model overnight. Or consider renting one for a few days. You and your family will get more of a feel for the car than you would with a 15-minute test drive.
Before you buy that car: Check the manufacturer's recommendation about oil. More are recommending expensive synthetic blends, which could noticeably add to the price of an oil change.
If you're thinking about buying a pre-owned car ask around at local mechanics (non-dealer affiliated) about common problems and the costs associated with them. Also check in with your local body shop to get a good idea of problem rust locations and possible ways to prevent it from starting. You may also want to take note of the paint color as some are much more expensive than others. This could mean higher insurance premiums and out-of-pocket expenses.
Consider the cost of insurance when purchasing a new/used car. Don't let the salesperson talk you into buying a car and then raising your deductible to afford what you want. This can lead to even bigger budget crunches down the road. It will cost you a little more each month but it could be your saving grace later. A good rule of thumb is that your deductible should not exceed the amount of money you have in a savings account at any given time. No one plans to have an accident; they just happen. I say "Fail to plan, Plan to Fail"
When purchasing a vehicle that has a salvaged title, remember that you will not be able to trade it in to a dealer at a later date. You must sell it on your own. Also, when buying a repairable vehicle remember that the damages to that vehicle exceeded 80% of the value of the vehicle. Unless you can do the work yourself it usually doesn't pay. Only buy a repaired salvage vehicle from a reputable place or take it to a local trusted body shop to assess the repairs before you purchase. Things can look ok cosmetically, but could be a danger to you or your family structurally if the repairs were not done properly.