Does you rMP3 unit have a separate hard drive to install? You could always put that in place of the CD changer.

Rims and tires... biggest I suggest is 18x9 with positive offset and 285/60 tires. Anything bigger will rub. If you want black, and don't mind a little extra $$$, go look for a set of wheels that appeals to you and have them powdercoated.

Use 87 octane. Higher octane won't give you any better performance or mileage. It won't hurt, but it will cost you more $$$