Yeah, that would work for the clay bar. Another person recommended the Meguiars ultimate compound. Also very good stuff. My Advice is to use a random orbit buffer and let the machine do the work. Pressing too hard either by hand or machine is going to open a pandoras box and just make swirls and haze a bigger issue. If you go clay bar, get a spray bottle with warm (almost hot) water and keep misting the body and bar keep it real wet.
From the looks of your picture.... the mass near the fender is probably a no go. Those scuffs look pretty knarly. Start out with light pressure and go a few rounds.... Clean it all off with 90% Isopropanol, let it dry and feel the surface again. Sorry to say but, I doubt you will have 100% success getting them all out. Good luck.