I try to shoot on the lowest F-stop I can get while still including what I want to focus on to be in focus. I won't always have the entire car completely in focus if I'm trying to place the attention on a specific area of the car. Giving the background a little bit of natural-looking blur can really give the photo depth, make the focus-area pop, and make the background less busy and distracting.
I really like these shots compared to the first batch, excluding the one where your friend walked into frame. You still have some shots where you're shooting on the shadow-side of the car, but that looks more like it was due to car placement limitations being next to the road like that. The head-on shots of the front-end could have been improved if you used the Rule of Thirds and put the car off to the left-side of the frame and included the bends of the road on the right. There is nothing wrong with having the car dead center in the frame, but using the Rule of Thirds can mix it up and add some setting interest into your shots. Also, don't be afraid to get low when taking shots. Your car is below you, get down to it's height sometimes, just like you did in the 2nd shot.
Look to see if the Nikkor 50mm f/1.8D lens will work with your camera, it should. I think it's the best bang-for-your-buck lens out there. It's a fixed lens, so there's no zoom capability. But the beautiful depth of field it provides, and incredible sharpness it's capable of makes it a must-have for a Nikon shooter. Price for one is around $100. I highly recommend it, I bought mine the day after I picked up my D200 a couple years back.
EDIT: I also wanted to add, that location wasn't the best for taking photos. Its better to have a bigger area, away from traffic and distractions. A place with more room to move around in.
