it is a value judgement. materialism is a relative term. i agree with your notion that if you truly enjoy something, such as cars, for the purpose they were intended (e.g. drive them like the devil because you appreciate the technology and performance), then you are not being materialistic. however, from the perspective of someone who is not interested in cars, it may appear materialistic TO THEM.
that said, whether you are materialistic or not...well, who gives a shit. i enjoy being a consumer, and i will buy things because i just damn feel like it. for example, if i see a watch i like, i'll buy it, even though i have two bell and ross automatics...so who cares? if you have money to spend, and consider yourself a half-way rational consumer, then buy whatever you want, and forget about whether you are being "materialistic" or not.
some of us are more vain than others. does looking into the mirror in the morning to make sure your hair isn't frizzy or standing up mean your are vain? how about dying your hair to cover up the grey? vain and narsissistic? maybe...but WHO CARES. life if meant to be enjoyed. if you masturbate 10 times a day or have sex with 5 different people in a day, does that make you a glutton or whore? maybe...but did you enjoy yourself while not at the expense of others? if so, then congratu-fucka-lations.