so wait your saying an Evo is not in your age bracket??? I know a few 18yr olds who have Evo9's and they're awesome. Insurance hurts the wallet and the mod bug has hit hard, but they are awesome cars.
AS for your question I would nver comapre the 350Z, M3 to an A4, they are a different mindset. I would compare a regular 3 series 4door to an A4 and would take the B6 over a comparable E46 3erIt depends on your budget too, All German cars will cost more to maintain than say Japanese/American cars will. All in all I got my Audi at 17, but thankfully my parents cover repairs

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I would say if you have a good job, or parents are taking care of the big items then an Audi is something you should def. look into.