Multisite with search functionality
I would not use a multisite for this.  Instead, I would do Option B (mysite.com/moto).

Then I would use the Themekey module to set up different themes.  If you want the two parts of the site to basically look the same, you can make the motorcycle part a subtheme of the theme you are using for the car part.

If you do this, then you don't have to worry about integrating the data; you will have two parts of the website that look different, but on the backend all the data will be kept together.