Another thing to consider is that the car buying process can be lengthy depending on what your demands are for the vehicle and timing. Having bought a car recently myself, patience is the name of the game. Their job is to move cars, so the longer it sits on the lot the better you can bargain. You might have found a dealer during a good month for them so they aren't as willing to bargain, but then they call you in 5 weeks when sales cool off asking if you're still interested and suddenly meet the price you want. Or the car you really want isn't in stock and you might have to wait until they get one from another dealership. It will be a detriment to you to be bleeding money renting a car while seeking another, especially if you tell the dealership that you're doing this (don't). On the flip side, you could get lucky and find your dream car at a dealership that's willing to work with you right away. It is more likely that the process will take longer if you are going to buy used because the amount of vehicles with a particular set of options you desire is going to be fewer, but USAA's car buying service can make it go faster.
Be aware that many dealerships will offer some kind of military incentive, and assuming that your credit was good before your employment in the military will make you eligible for the lowest financing rates they offer.
USAA offers 1.9% apr, 1.49 if you use their car buying service, so there's your starting point to ask the dealership to beat that APR in financing. It's pretty neat, although it doesn't always list the option packages. You put in the car you want and participating dealers will offer their bottom line price with no haggling required on your part. For example, one of the cars I looked at was the Ford Fusion, and the average savings through USAA's car buying service was $3-5k off sticker price. However, I did discover that dealerships don't always have the exact car that you listed in USAA's car buying service, but they will call you just to get you into the dealership. You hand them the piece of paper USAA generates and then they respond with "oh, we don't have that" but they did offer the same bottom-line price on a similarly equipped car. That was pretty frustrating to me considering the participating dealerships were 1/2-1 hour away, so when they call you just tell them over the phone what you are looking for and what concessions, if any, you're willing to make before you trek to the dealership.
Navy Fed offers similar car loan APRs if you have a checking account with them.
Your best bet, if you can swing it, is to buy a cheap used car until you get the car you want, then sell the beater at little or no loss to you.
