re: What's a smart down payment for a $25-$30k vehicle?

If you're going to a dealer to purchase vehicle make sure you know what interest rate your bank/credit union will charge you. Interest rate is negotiable at dealership and they won't voluntarily give you the best rate they can.

I wouldn't tell the car salesman that your putting any money down until after you finalize sales price.

Also, it's in your best interest to do a private party sale for your current vehicle.

And FINALLY, make sure you walk-out on the deal. They'll call you the next day with a better offer.