If you use the wider spray pattern and avoid lingering on electrical connections you should be fine. Use the engine degreaser for the sensitive areas to help break up the junk and just get after it on the frame and suspension areas.
when it comes to rust, unless you remove the cancer it will come back. Now, if its just little surface rust you may get away with POR15 for quite a while but its still there. Although I dont think POR15 is meant to be painted to a nice finish like on a body. If its underside you will b fine.