I have several really small nicks and major scuffs on my car; the nicks from driving on the highway (flying debris hitting chassis) and the scuffs from two fender benders. The nicks have started to rust and the scuffs are just annoying, so I'd like to get rid of them. A body repair shop charges me for each body piece, so that doesn't make for a good deal. I was wondering if I can resolve it with something like DAP Touch'n'Tone multipurpose spray paint?
You can, but probably not in the way you're thinking. You can't just go out and buy a can of "red spray paint", unless you are prepared to re-paint the whole car in that color or you really don't care about the results. This is because your car isn't "red" (for example), it's "Sierra Red (L D3V)".
However, if you go to a professional paint supply shop you can get them to inject the specific color for your car into a spray can. You can often get "touch up" bottles of paint in the exact color from your dealer.
The rust is another matter though... If it's surface rust, you may be able to get away with just using primer that reacts with and neutralizes the rust, and then paint over that. However, if the rust is "flaking" or working behind other parts of the paint, you're probably going to have to clean that out, for example with a wire wheel on a tool, possibly making the area you have to paint much larger. It's either that or risk having to deal with a larger area (or a hole clean through) later.
These rust-reacting primers, like certain models of Rustoleum, work extremely well in my experience. When I was younger I had a couple of cars that had varying degrees of paint damage near the wheel wells or on the front end, and was very happy with the rust-stopping results of Rustoleum.
In my experience though, if you want something that looks good, you need to do the whole panel. And if you want something that looks really good, you have to do it with the right tools in a booth.
Consider going to a detailer. It's amazing how much they can do without actually painting anything.
You can buy touch up paint that matches the paint color of your car. This can be used to fill in the nicks in the paint.
Not sure if this would help with the scuffs, depends on how large and deep they are.
Normal spray paint may likely chip off in a few days.
You should be able to buy colour-matched automotive spray paint for your car in most larger car-parts shops (in the UK, Halfords sell a wide range, and their larger stores can match considerably more from manufacturer's paint codes).
However, as others have said, it can be very tricky to get a good match as your paint will have faded, and the new will be matched to the original factory colour. You might be able to get away with it by using t-cut or similar to restore the original paint (they effectively work by removing the top layer of the paint, so don't try it too often!), then carefully blending in the new paint, but it's not easy - it depends how good you want the result to look!
If you've got scratches strarting to rust you'll need to take them back to bare metal and paint with an etch primer first, otherwise the rust will spread under the new paint.