I have an older digital Canon Rebel SLR and I am no expert on photography, but you can really tell a difference in the pictures. If you get into it, you will find that you will need to buy at least two lens and then the one that comes with the camera will never be used again. I ended up forking over money for a EF 24-105mm f/4L IS USM and use it as my all around lens. It takes good close up and decent distance shots. I also use a EF 70-200mm f/4L IS USM, but looking back I would have probably rather have a wide angle or macro lens in my arsenal. It takes good pictures, but I just don't use it much.
Lens are not cheap! They can and usually do cost more than the camera! Which you probably already know if you used to have a 300mm and 400mm lens.
For taking pictures of models, I would recommend getting a camera without the lens (used to be able to find those kits online), and invest in a macro lens for taking picture of objects that are close.
As for taking pictures of real cars, what ever lens you use, add a polarized filter to it! It is amazing what cutting the suns reflection off of windows, chrome and car bodies will do to add snap and clarity to your pictures. Usually those can be rotated to get the effect or remove it.
As far as manufactures, Canon and Nikon seem like they are tops for ease of use combined with quality. I think you would be happy with either of their entry models which run right at msrp of $500. Mine is an older 8 mega pixel camera and I am more than happy with it. With that resolution and higher, it makes it easy to go back later and zoom and crop the picture to get exactly what you are after. The higher the resolution, the more you can reach into a picture and do that. Most new cameras now start with at least 10 and 12 mega pixel.
If you are looking at spending only $500, you might consider Craigslist or local ads for a used camera and you might be able to score a good lens or two with it.
Not sure how much help that was, but hopefully you can get a little bit out of it.
Here is a sample (it has been compressed from the original) that shows what the polarized filter will do. If the filter was not on you wouldn't see the windshield so clearly and the glare would much greater.
Oh yeah, by going digital, all of your unaltered images will have information imbedded in the file that give you detail as to what camera was used, what lens, focal length, etc... Kind of cool to have.