Good start......carefully and it looks, accurately marking off what you want to change. I'd seriously recommend you get a set of small files if you don't already have some. The control you get is much better than sandpaper because they're rigid and you can make them cut exactly what you want. The 'must-have' set is about 7" long including handle, in several shapes...flat, trianglular, round, etc.....and usually comes in a little plastic organizer sleeve. Like this....
I'd also recommend you get a set of jeweler's files, which are smaller for very fine detail work, and a set of riffler files, which have curved ends for getting into tight places.
I do a lot of heavy mods and usually make my rough cuts with a Dremel, then take the edge down to the mark with files and finally, just remove the file-marks with 400 grit or finer sandpaper.
If you don't have files or are in a hurry, 80, 100 or 180 grit sandpaper glued to something rigid like a popsickle stick will work as a Q&D file, as will small fingernail files, but without as much control of the cut.
For removing emblems and trim, I also prefer to start with files. A crosscut file will machine just the emblem or other detail you're removing flush with the surface, whereas sandpaper that's course enough to remove an emblem will also make a lot of deep scratches in the surrounding plastic, causing you extra work to remove them. Using a finer grit to remove an emblem or other small detail will tend to make waves in the surrounding material before it actually cuts the detail flush with the surface. Again, nail files or sandpaper glued to something rigid will work, but files make life much easier.
I got sets of all three types at a cheap tool outlet , and though the Chinese steel is soft, it will do a good job on plastic. The teeth of the files will tend to clog as you use them, but an occasional brushing-out with a brass-bristle brush, or even an old toothbrush, will keep them clean and free-cutting.
If you can't find a kit bumper that's a good starting point, scratch-building one is straightforward. Just draw the shape (being careful of symmetry) you want on a sheet of styrene of the correct thickness and cut it out with a razor saw and X-acto knives, fit it carefully to the front body contours, then taper the front edges of the bumper blade with your files, primer and Alclad it. Make the lower grille guards the same way.
Moving the parking lights requires removing the raised material from around the existing holes, filling the remaining holes with flat styrene and surface-filling to shape. Make symmetrical templates of the openings in the front panel for your new lights, and open the new holes with small drills, knives and files. If you make really clean and sharp openings for the new lights, it shouldn't be too hard to find some lenses with a useable pattern already in them, and cut them down to fit the openings.