Remember if you trade in your vehicle, you're going to get about half of what you would if you sold it outright. Need to consider that when you look at what your loan payoff is
As for what you should get - get thee to your bank. Sit down with the loan advisor, and pull your credit and the current loan. Find out what the payoff is. Find out what a dealership will likely get you.
Find out what the bank will lend you - knowing you may be upside down (they will tell you that figure). From there, you will have a dollar amount, and you can start looking at vehicles. Then you can know what you can afford.
Never buy brand new. Driving off the lot and knowing you just left $10,000 sitting in there is a not good feeling.
I purchased my most recent vehicle from a local small dealer, with a reputation of performing a good check on the vehicles, and selling good used cars. If the cars aren't to their quality, they take them to auction to let someone else sell them. I actually asked my loan advisor who she recommended, it was this dealer. I now drive a 1997 Expedition with 154k miles, and I'm thrilled to death with it. I paid $1200 less than book price for it. I actually work with the person who traded it in, and they already miss it
My loan is only three years, and payments are under $150 a month, plus it's ridiculously cheap to insure for some reason.
Buying a car is a LOT LOT LOT easier when you start looking with a loan already in hand. You know your range, and once you find something, it's just calling your bank, giving them the VIN (they verify it's worth what you are paying on), getting a check from them, getting insurance, and going to the dealer and driving off. No BS, no crazy interest rates, no wondering if you're getting scammed.