1. Strength does vary quite considerably, as does things like resistance to heat and UV, which are important for things you'd stick to a car. PLA is pretty much out of the question, as the temperature at which it becomes soft is too low when you consider it will be attached to hot metal in the summertime. ABS is better and is less "brittle" than PLA, so is probably a better choice. Nylon is another alternative, is UV-resistant and better at higher temperatures, but is more expensive. Painting the ABS would deal with the UV issue, though.

2. A car body has already been printed - http://en.wikipedia.org/wiki/Strati_%28automobile%29, but I would question how well it would last long-term. If you look close-up, the finish of the body is extremely rough. This is a direct consequence of printing large things in a (relatively) small amount of time. It takes time to 3D-print - the larger the print, the more time it takes. You can reduce the time by reducing the quality (or layer height) of the print, but your finish will not be as good. You also need to consider the weight of what you're printing. To get any strength from the finished article, your bodykit panels will be significantly thicker than their fiberglass equivalent. For that reason, you would be better off printing the panel, filling and smoothing it, creating a mold from the positive then using that mold to create your panels from something stronger, cheaper and more lightweight (also much faster to produce one your molds are done).

3. Can't help you here. I'm not aware of any service in the UK that can print parts to the size you're after. A word of warning, though: anyone you find that can print something that bid is going to charge you in the order of thousands of pounds for a single panel.