Thank you very much for your replies, guys. Yes, this is actually my 5th model. I've built 3 submarines and one plane before this. I will stick to ships and planes from now on, but this was a nice model to gain some experience.
The paint he used was spray can black enamel and he had some serious overspray some places. I initially tried to sand it out, but that ended up ruining other details and it was a PITA. Then I used lacquer/enamel thinner and a toothbrush. I also used 91% alcohol. Alcohol is good for cleaning up after the above but normally isn't strong enough to cut through some thicker paint deposits.
The problem with lacquer/enamel thinner is that it softens the plastic and so the hood ended up braking in about 4-5 places (I hate that damn hood!) and the body of the car in about 6 places (mainly thinner details such as top pillars and rear side where the tail lights went in.
I have since purchased some Gunze Sangyo paint stripping fluid but I have not tried it yet.
Now as far as darkened lines, I have successfully used this technique on my previous model (pardon the canopy):

but I know my friend doesn't really like weathered stuff, although now I realize that I could have done it and he wouldn't even notice it since this is technically not weathering, but accenting some lines. So that's something to keep in mind if I decide to build another car in the future. Another issue was that the lines were not that deep anymore after tons of primer and such (to fix flaws from stripping/sanding) and from my limited experience, washes tend to flow better in slightly deeper lines.
As for the tires, I actually have a Gunze Sangyo flat "Tire" acrylic bottle that I could have sprayed them with, but I did not think of it....
I think the only reason I've made progress with my few models is that I've SERIOUSLY developed my patience with this hobby so I took the time with them, but apparently I didn't take enough to notice these other smaller details. I do very much appreciate all your comments and criticism as this is the only way I've learned how to build these things.
My next kit is another german u-boat, an XXI 1/144 scale, although I *might* build another car for him for next new year's (I swore this was the last car I was going to build but he told me he's gonna make him build a 'Cuda too

).
Thanks again for all the comments!