I would say lease the redesigned 3-series sedan or coupe if you can't upgrade to anything else. my parents bought me the new one for my 16th birthday (I don't have it anymore but only because I wanted an SUV instead) and it was really an awesome car. it handles unbelievably well, the sound system is impressive for an entry-level car like that, and I think the interior was much, much nicer looking than even my Range Rover's. it's an all-around attractive car, much better than any of its counterparts IMO.
also, please don't get a Z. I hate them. I just think they're infinitely ugly. :X
if you could spend a bit more then get a 5-series instead, the lowest-model 5 is actually not THAT much more expensive than a 3, plus it comes with navigation and some other stuff standard I think. also, like someone said, the Cayman would be a good choice as well, although it costs a bit more.