Why does Grenouille begin and end his killing spree with redheaded girls?

The fact that he ends his killing spree with another redhead isn't something that is planned. She was only a child, and he found her by sense of smell..... her looks had nothing to do with the reasons he chose her.

For your second question, the excerpt below should help greatly;

Theory of the sovereignty of scent

This novel takes as a premise that scent controls a large portion of human behavior, usually on an unconscious level. It is important to note this premise, for the entire internal plot (but not necessarily the external plot of Grenouille's social actions) turns on this idea. It is not only his supernatural sense of smell that is the focus of Grenouille's life, but the idea that humans' scents are integral to their humanity. Grenouille is subhuman, both in his own mind and, at least unconsciously, in the minds of others, because he has no personal odor. When he discovers this personal characteristic in his hideout in the Massif Centrale, he is shocked and somewhat horrified. He has never met another human being with no smell; that he cannot smell himself, despite his marvelous nose, seems monstrous to him--demonstrating why he seems monstrous to everyone else.

A corollary to the premise that scent is nearly tyrannical--determining a great deal of how people treat each other--is that adolescent girls have the best scents. This idea is further refined with the perception that beautiful girls have better scents than other girls, and with those of the red-haired type having the finest. It is also maintained that these teenage female scents are appealing to everyone, not simply heterosexual males. This last idea is perhaps the most fantastical notion of all. It creates the possibility of the ending, in which Grenouille, drenched with the scents of the dead girls, becomes so appealing that the Paris mob eats him.

I don't believe the red hair is a fetish; he is looking for particular scents, not hair color.