Can guys get an erection from simply seeing a (fully-clothed) good-looking girl?

Yes. If she has nice hips, and she's wearing tight jeans, if I can see the shape of her hips, it can happen to me!
I love nice hips. ;-)

Does she have to be good-looking too?

Yes, guys can get erections at a funeral, much less looking at a girl.

Yes. Men and Women think completely differently. While most women are turned on by how they FEEL, or what they are thinking; most men are turned on visually. A man can see a woman and go directly to thoughts of sex whereas a woman thinks about the probability of that man's success, confidence, and health, among other things which then leads to thoughts about sex. This is why most guys like pornography and why most women don't.

Yup.

Yep, and don't be surprised if they cum too! :)

For me it depends on the kind of clothes she is wearing.. and how curvy she is.. but yes

Yes. some guys get an erection very easily. And others start to fantasize and, presto, instant erection.

Yes sorry to say but yes...

Yes. they can fantasize