Welcome, Spencer - another 87 300E owner. The taillights only need the lens replaced, but you may have to play with installing an orange bulb for the rear turn indicators, depending on local laws. Lens replacement is easy, you just remove the screws holding the old lens in place (inside the trunk, behind the tail light bulb access hatches).
A rear spoiler has a real function on the 300E - it makes it easier to judge the rear of the car when parking or backing up. Otherwise, it is a matter of personal taste.
Before you go to the expense of side skirts (Big $, almost as much as a Sportline handling kit or aftermarket lowering springs, sway bars, and shocks), some members have just had the bumpers, rocker panels, and side impact strip painted to match the body paint, and it looks really nice (I would leave the black bumper inserts black if I were doing it just to minimize rock dings). AV on this forum did that, his car is below:
Also, buying a new plastic grill insert, painting it matte black (not flat or glossy, half way in between) and installing new aluminum trim strips really cleans up the front as well. The trim strips from a late model C230 can be adapted for a slightly bolder chrome strip look, as they are a bit more pronounced than the 87 strips.
Removing the 300E symbol on the trunk is another easy mod that cleans up the back and makes the car look wider and newer as well, and is in the archives.
17" wheels are a matter of preference - the 300E has been said to handle best with 15-16" wheels, as the 17's add unsprung weight and slightly decrease acceleration, and make the ride stiff and the steering tramline a bit - they look nice though.
There should be threads in the archives on all these mods, and you can see some of them on my car by clicking the signature line below.
Good luck, and have fun!